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Table of contents.

• Experiment 1 - Magnetic Fields of Coils and Faraday's Law
• Experiment 2 - Microwave Optics
• Experiment 3 - Geometrical Optics
• Experiment 4 - Physical Optics
• Experiment 5 - Fluids and Thermodynamics

Experiment 6 - The Photoelectric Effect

• Experiment 7 - Radioactivity
• Photodiode with amplifier
• Batteries to operate amplifier and provide reverse voltage
• Digital voltmeter to read reverse voltage
• Source of monochromatic light beams to irradiate photocathode
• Neutral filter to vary light intensity

INTRODUCTION

The energy quantization of electromagnetic radiation in general, and of light in particular, is expressed in the famous relation

\begin{eqnarray} E &=& hf, \label{eqn_1} \end{eqnarray}

where \(E\) is the energy of the radiation, \(f\) is its frequency, and \(h\) is Planck's constant (6.63×10 -34 Js). The notion of light quantization was first introduced by Planck. Its validity is based on solid experimental evidence, most notably the photoelectric effect . The basic physical process underlying this effect is the emission of electrons in metals exposed to light. There are four aspects of photoelectron emission which conflict with the classical view that the instantaneous intensity of electromagnetic radiation is given by the Poynting vector \(\textbf{S}\):

\begin{eqnarray} \textbf{S} &=& (\textbf{E}\times\textbf{B})/\mu_0, \label{eqn_2} \end{eqnarray}

with \(\textbf{E}\) and \(\textbf{B}\) the electric and magnetic fields of the radiation, respectively, and μ 0 (4π×10 -7 Tm/A) the permeability of free space. Specifically:

No photoelectrons are emitted from the metal when the incident light is below a minimum frequency, regardless of its intensity. (The value of the minimum frequency is unique to each metal.)

Photoelectrons are emitted from the metal when the incident light is above a threshold frequency. The kinetic energy of the emitted photoelectrons increases with the frequency of the light.

The number of emitted photoelectrons increases with the intensity of the incident light. However, the kinetic energy of these electrons is independent of the light intensity.

Photoemission is effectively instantaneous.

Consider the conduction electrons in a metal to be bound in a well-defined potential. The energy required to release an electron is called the work function \(W_0\) of the metal. In the classical model, a photoelectron could be released if the incident light had sufficient intensity. However, Eq. \eqref{eqn_1} requires that the light exceed a threshold frequency \(f_{\textrm{t}}\) for an electron to be emitted. If \(f > f_{\textrm{t}}\), then a single light quantum (called a photon ) of energy \(E = hf\) is sufficient to liberate an electron, and any residual energy carried by the photon is converted into the kinetic energy of the electron. Thus, from energy conservation, \(E = W_0 + K\), or

\begin{eqnarray} K &=& (1/2)mv^2 = E - W_0 = hf - W_0. \label{eqn_3} \end{eqnarray}

When the incident light intensity is increased, more photons are available for the release of electrons, and the magnitude of the photoelectric current increases. From Eq. \eqref{eqn_3}, we see that the kinetic energy of the electrons is independent of the light intensity and depends only on the frequency.

The photoelectric current in a typical setup is extremely small, and making a precise measurement is difficult. Normally the electrons will reach the anode of the photodiode, and their number can be measured from the (minute) anode current. However, we can apply a reverse voltage to the anode; this reverse voltage repels the electrons and prevents them from reaching the anode. The minimum required voltage is called the stopping potential \(V_{\textrm{s}}\), and the “stopping energy” of each electron is therefore \(eV_{\textrm{s}}\). Thus,

\begin{eqnarray} eV_{\textrm{s}} &=& hf - W_0, \label{eqn_4} \end{eqnarray}

\begin{eqnarray} V_{\textrm{s}} &=& (h/e)f - W_0/e. \label{eqn_5} \end{eqnarray}

Eq. \eqref{eqn_5} shows a linear relationship between the stopping potential \(V_{\textrm{s}}\) and the light frequency \(f\), with slope \(h/e\) and vertical intercept \(-W_0/e\). If the value of the electron charge \(e\) is known, then this equation provides a good method for determining Planck's constant \(h\). In this experiment, we will measure the stopping potential with modern electronics.

THE PHOTODIODE AND ITS READOUT

The central element of the apparatus is the photodiode tube. The diode has a window which allows light to enter, and the cathode is a clean metal surface. To prevent the collision of electrons with air molecules, the diode tube is evacuated.

The photodiode and its associated electronics have a small “capacitance” and develop a voltage as they become charged by the emitted electrons. When the voltage across this “capacitor” reaches the stopping potential of the cathode, the voltage difference between the cathode and anode (which is equal to the stopping potential) stabilizes.

To measure the stopping potential, we use a very sensitive amplifier which has an input impedance larger than 10 13 ohms. The amplifier enables us to investigate the minuscule number of photoelectrons that are produced.

It would take considerable time to discharge the anode at the completion of a measurement by the usual high-leakage resistance of the circuit components, as the input impedance of the amplifier is very high. To speed up this process, a shorting switch is provided; it is labeled “Push to Zero”. The amplifier output will not stay at 0 volts very long after the switch is released. However, the anode output does stabilize once the photoelectrons charge it up.

There are two 9-volt batteries already installed in the photodiode housing. To check the batteries, you can use a voltmeter to measure the voltage between the output ground terminal and each battery test terminal. The battery test points are located on the side panel. You should replace the batteries if the voltage is less than 6 volts.

THE MONOCHROMATIC LIGHT BEAMS

This experiment requires the use of several different monochromatic light beams, which can be obtained from the spectral lines that make up the radiation produced by excited mercury atoms. The light is formed by an electrical discharge in a thin glass tube containing mercury vapor, and harmful ultraviolet components are filtered out by the glass envelope. Mercury light has five narrow spectral lines in the visible region — yellow, green, blue, violet, and ultraviolet — which can be separated spatially by the process of diffraction. For this purpose, we use a high-quality diffraction grating with 6000 lines per centimeter. The desired wavelength is selected with the aid of a collimator, while the intensity can be varied with a set of neutral density filters. A color filter at the entrance of the photodiode is used to minimize room light.

The equipment consists of a mercury vapor light housed in a sturdy metal box, which also holds the transformer for the high voltage. The transformer is fed by a 115-volt power source from an ordinary wall outlet. In order to prevent the possibility of getting an electric shock from the high voltage, do not remove the cover from the unit when it is plugged in.

To facilitate mounting of the filters, the light box is equipped with rails on the front panel. The optical components include a fixed slit (called a light aperture) which is mounted over the output hole in the front cover of the light box. A lens focuses the aperture on the photodiode window. The diffraction grating is mounted on the same frame that holds the lens, which simplifies the setup somewhat. A “blazed” grating, which has a preferred orientation for maximal light transmission and is not fully symmetric, is used. Turn the grating around to verify that you have the optimal orientation.

The variable transmission filter consists of computer-generated patterns of dots and lines that vary the intensity of the incident light. The relative transmission percentages are 100%, 80%, 60%, 40%, and 20%.

INITIAL SETUP

Your apparatus should be set up approximately like the figure above. Turn on the mercury lamp using the switch on the back of the light box. Swing the \(h/e\) apparatus box around on its arm, and you should see at various positions, yellow green, and several blue spectral lines on its front reflective mask. Notice that on one side of the imaginary “front-on” perpendicular line from the mercury lamp, the spectral lines are brighter than the similar lines from the other side. This is because the grating is “blazed”. In you experiments, use the first order spectrum on the side with the brighter lines.

Your apparatus should already be approximately aligned from previous experiments, but make the following alignment checks. Ask you TA for assistance if necessary.

Check the alignment of the mercury source and the aperture by looking at the light shining on the back of the grating. If necessary, adjust the back plate of the light-aperture assembly by loosening the two retaining screws and moving the plate to the left or right until the light shines directly on the center of the grating.

With the bright colored lines on the front reflective mask, adjust the lens/grating assembly on the mercury lamp light box until the lines are focused as sharply as possible.

Roll the round light shield (between the white screen and the photodiode housing) out of the way to view the photodiode window inside the housing. The phototube has a small square window for light to enter. When a spectral line is centered on the front mask, it should also be centered on this window. If not, rotate the housing until the image of the aperture is centered on the window, and fasten the housing. Return the round shield back into position to block stray light.

Connect the digital voltmeter (DVM) to the “Output” terminals of the photodiode. Select the 2 V or 20 V range on the meter.

Press the “Push to Zero” button on the side panel of the photodiode housing to short out any accumulated charge on the electronics. Note that the output will shift in the absence of light on the photodiode.

Record the photodiode output voltage on the DVM. This voltage is a direct measure of the stopping potential.

Use the green and yellow filters for the green and yellow mercury light. These filters block higher frequencies and eliminate ambient room light. In higher diffraction orders, they also block the ultraviolet light that falls on top of the yellow and green lines.

PROCEDURE PART 1: DEPENDENCE OF THE STOPPING POTENTIAL ON THE INTENSITY OF LIGHT

Adjust the angle of the photodiode-housing assembly so that the green line falls on the window of the photodiode.

Install the green filter and the round light shield.

Install the variable transmission filter on the collimator over the green filter such that the light passes through the section marked 100%. Record the photodiode output voltage reading on the DVM. Also determine the approximate recharge time after the discharge button has been pressed and released.

Repeat steps 1 – 3 for the other four transmission percentages, as well as for the ultraviolet light in second order.

Plot a graph of the stopping potential as a function of intensity.

PROCEDURE PART 2: DEPENDENCE OF THE STOPPING POTENTIAL ON THE FREQUENCY OF LIGHT

You can see five colors in the mercury light spectrum. The diffraction grating has two usable orders for deflection on one side of the center.

Adjust the photodiode-housing assembly so that only one color from the first-order diffraction pattern on one side of the center falls on the collimator.

For each color in the first order, record the photodiode output voltage reading on the DVM.

For each color in the second order, record the photodiode output voltage reading on the DVM.

Plot a graph of the stopping potential as a function of frequency, and determine the slope and the \(y\)-intercept of the graph. From this data, calculate \(W_0\) and \(h\). Compare this value of \(h\) with that provided in the “Introduction” section of this experiment.

Procedure Part 1:

Photodiode output voltage reading for 100% transmission =                                              

Approximate recharge time for 100% transmission =                                              

Photodiode output voltage reading for 80% transmission =                                              

Approximate recharge time for 80% transmission =                                              

Photodiode output voltage reading for 60% transmission =                                              

Approximate recharge time for 60% transmission =                                              

Photodiode output voltage reading for 40% transmission =                                              

Approximate recharge time for 40% transmission =                                              

Photodiode output voltage reading for 20% transmission =                                              

Approximate recharge time for 20% transmission =                                              

Photodiode output voltage reading for ultraviolet light =                                              

Approximate recharge time for ultraviolet light =                                              

Plot the graph of stopping potential as a function of intensity using one sheet of graph paper at the end of this workbook. Remember to label the axes and title the graph.

Procedure Part 2:

First-order diffraction pattern on one side of the center:

Photodiode output voltage reading for yellow light =                                              

Photodiode output voltage reading for green light =                                              

Photodiode output voltage reading for blue light =                                              

Photodiode output voltage reading for violet light =                                              

Second-order diffraction pattern on the other side of the center:

Plot the graph of stopping potential as a function of frequency using one sheet of graph paper at the end of this workbook. Remember to label the axes and title the graph.

Slope of graph =                                              

\(y\)-intercept of graph =                                              

\(W_0\) =                                              

\(h\) =                                              

Percentage difference between experimental and accepted values of \(h\) =                

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Photoelectric Effect: Definition, Equation & Experiment

Everything learned in classical physics was turned on its head as physicists explored ever smaller realms and discovered quantum effects. Among the first of these discoveries was the photoelectric effect. In the early 1900s, the results of this effect failed to match classical predictions and were only explainable with quantum theory, opening up a whole new world for physicists.

Today, the photoelectric effect has many practical applications as well. From medical imaging to the production of clean energy, the discovery and application of this effect now has implications that go well beyond simply understanding the science.

What Is the Photoelectric Effect?

When light, or electromagnetic radiation, hits a material such as a metal surface, that material sometimes emits electrons, called ​ photoelectrons ​. This is essentially because the atoms in the material are absorbing the radiation as energy. Electrons in atoms absorb radiation by jumping to higher energy levels. If the energy absorbed is high enough, the electrons leave their home atom entirely.

This process is sometimes also called ​ photoemission ​ because incident photons (another name for particles of light) are the direct cause of the emission of electrons. Because electrons have a negative charge, the metal plate from which they were emitted is left ionized.

What was most special about the photoelectric effect, however, was that it did not follow classical predictions. The way in which the electrons were emitted, the number that were emitted and how this changed with intensity of light all left scientists scratching their heads initially.

Original Predictions

The original predictions as to the results of the photoelectric effect made from classical physics included the following:

• Energy transfers from incident radiation to the electrons. It was assumed that whatever energy is incident upon the material would be directly absorbed by the electrons in the atoms, regardless of wavelength. This makes sense in the classical mechanics paradigm: Whatever you pour into the bucket fills the bucket by that amount.
• Changes in light intensity should yield changes in kinetic energy of electrons. If it is assumed that electrons are absorbing whatever radiation is incident upon them, then more of the same radiation should give them more energy accordingly. Once the electrons have left the bounds of their atoms, that energy is seen in the form of kinetic energy.
• Very low-intensity light should yield a time lag between light absorption and emission of electrons. This would be because it was assumed that electrons must gain enough energy to leave their home atom, and low-intensity light is like adding energy to their energy “bucket” more slowly. It takes longer to fill, and hence it should take longer before the electrons have enough energy to be emitted.

Actual Results

The actual results were not at all consistent with the predictions. This included the following:

• Electrons were released only when the incident light reached or exceeded a threshold frequency. No emission occurred below that frequency. It didn’t matter if the intensity was high or low. For some reason, the frequency, or wavelength of the light itself, was much more important. 
• Changes in intensity did not yield changes in kinetic energy of electrons. They changed only the number of electrons emitted. Once the threshold frequency was reached, increasing the intensity did not add more energy to each emitted electron at all. Instead, they all ended up with the same kinetic energy; there were just more of them.
• There was no time lag at low intensities. There seemed to be no time required to “fill the energy bucket” of any given electron. If an electron was to be emitted, it was emitted immediately. Lower intensity had no effect on kinetic energy or lag time; it simply resulted in fewer electrons being emitted. 

Photoelectric Effect Explained

The only way to explain this phenomenon was to invoke quantum mechanics. Think of a beam of light not as a wave, but as a collection of discrete wave packets called photons. The photons all have distinct energy values that correspond to the frequency and wavelength of the light, as explained by wave-particle duality.

In addition, consider that the electrons are only able to jump between discrete energy states. They can only have specific energy values, but never any values in between. Now the observed phenomena can be explained as follows:

• Electrons are released only when they absorb very specific sufficient energy values. Any electron that gets the right energy packet (photon energy) will be released. None are released if the frequency of the incident light is too low regardless of intensity because none of the energy packets are individually big enough. 
• Once the threshold frequency is exceeded, increasing intensity only increases the number of electrons released and not the energy of the electrons themselves because each emitted electron absorbs one discrete photon. Greater intensity means more photons, and hence more photoelectrons. 
• There is no time delay even at low intensity as long as the frequency is high enough because as soon as an electron gets the right energy packet, it is released. Low intensity only results in fewer electrons.

The Work Function

One important concept related to the photoelectric effect is the work function. Also known as electron-binding energy, it is the minimum energy needed to remove an electron from a solid.

The formula for the work function is given by:

Where ​ -e ​ is the electron charge, ​ ϕ ​ is the electrostatic potential in the vacuum nearby the surface and ​ E ​ is the Fermi level of electrons in the material.

Electrostatic potential is measured in volts and is a measure of the electric potential energy per unit charge. Hence the first term in the expression, ​ -eϕ ​, is the electric potential energy of an electron near the surface of the material.

The Fermi level can be thought of as the energy of the outermost electron when the atom is in its ground state.

Threshold Frequency

Closely related to the work function is the threshold frequency. This is the minimum frequency at which incident photons will cause the emission of electrons. Frequency is directly related to energy (higher frequency corresponds to higher energy), hence why a minimum frequency must be reached.

Above the threshold frequency, the kinetic energy of the electrons depends on the frequency and not the intensity of the light. Basically the energy of a single photon will be transferred entirely to a single electron. A certain amount of that energy is used to eject the electron, and the remainder is its kinetic energy. Again, a greater intensity just means more electrons will be emitted, not that those emitted will have any more energy.

The maximum kinetic energy of emitted electrons can be found via the following equation:

Where ​ K max ​ is the maximum kinetic energy of the photoelectron, ​ h ​ is Planck's constant = 6.62607004 ×10 -34 m 2 kg/s, ​ f ​ is the frequency of the light and ​ f 0 ​ is the threshold frequency.

Discovery of the Photoelectric Effect

You can think of the discovery of the photoelectric effect as happening in two stages. First, the discovery of the emission of photoelectrons from certain materials as a result of incident light, and second, the determination that this effect does not obey classical physics at all, which led to many important underpinnings of our understanding of quantum mechanics.

Heinrich Hertz first observed the photoelectric effect in 1887 while performing experiments with a spark gap generator. The setup involved two pairs of metal spheres. Sparks generated between the first set of spheres would induce sparks to jump between the second set, thus acting as transducer and receiver. Hertz was able to increase the sensitivity of the setup by shining light on it. Years later, J.J. Thompson discovered that the increased sensitivity resulted from the light causing the electrons to be ejected.

While Hertz’s assistant Phillip Lenard determined that the intensity did not affect the kinetic energy of the photoelectrons, it was Robert Millikan who discovered the threshold frequency. Later, Einstein was able to explain the strange phenomenon by assuming the quantization of energy.

Importance of the Photoelectric Effect

Albert Einstein was awarded the Nobel Prize in 1921 for his discovery of the law of the photoelectric effect, and Millikan won the Nobel Prize in 1923 also for work related to understanding the photoelectric effect.

The photoelectric effect has many uses. One of those is that it allows scientists to probe the electron energy levels in matter by determining the threshold frequency at which incident light causes emission. Photomultiplier tubes making use of this effect were also used in older television cameras.

A very useful application of the photoelectric effect is in the construction of solar panels. Solar panels are arrays of photovoltaic cells, which are cells that make use of electrons ejected from metals by solar radiation to generate current. As of 2018, nearly 3 percent of the world’s energy is generated by solar panels, but this number is expected to grow considerably over the next several years, especially as the efficiency of such panels increases.

But most important of all, the discovery and understanding of the photoelectric effect laid the groundwork for the field of quantum mechanics and a better understanding of the nature of light.

Photoelectric Effect Experiments

There are many experiments that can be performed in an introductory physics lab to demonstrate the photoelectric effect. Some of these are more complicated than others.

A simple experiment demonstrates the photoelectric effect with an electroscope and a UV-C lamp providing ultraviolet light. Place negative charge on the electroscope so that the needle deflects. Then, shine the UV-C lamp. Light from the lamp will release electrons from the electroscope and discharge it. You can tell this happens by seeing the needle’s deflection reducing. Note, however, that if you tried the same experiment with a positively charged electroscope, it wouldn’t work.

There are many other possible ways to experiment with the photoelectric effect. Several setups involve a photocell consisting of a large anode that, when hit with incident light, will release electrons that are picked up by a cathode. If this setup is connected to a voltmeter, for example, the photoelectric effect will become apparent when shining the light creates a voltage.

More complex setups allow for more accurate measurement and even allow you to determine the work function and threshold frequencies for different materials. See the Resources section for links.

Related Articles

• Physics Hypertextbook: Photoelectric Effect
• Georgia State University: HyperPhysics: Photoelectric Effect
• UTK: Lab 2: The Photoelectric Effect
• UCLA Physics and Astronomy: Experiment 6 – The Photoelectric Effect
• Amrita: Photoelectric Effect

About the Author

Gayle Towell is a freelance writer and editor living in Oregon. She earned masters degrees in both mathematics and physics from the University of Oregon after completing a double major at Smith College, and has spent over a decade teaching these subjects to college students. Also a prolific writer of fiction, and founder of Microfiction Monday Magazine, you can learn more about Gayle at gtowell.com.

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Experimental physics i & ii "junior lab", photoelectric effect, description.

Photoelectric effect experiment equipment. (Image courtesy of MIT Junior Lab staff.)

The maximum kinetic energy of electrons ejected from a metal surface by monochromatic light is measured for several wavelengths. The value of Planck’s constant, h, is derived by an analysis of the data in the light of Einstein theory of the photoelectric effect.

Photoelectric Effect Lab Guide (PDF)

Planck, Max. Nobel Prize Lecture, “ The Genesis and Present State of Development of the Quantum Theory .” (1918).

Einstein, Albert. Nobel Prize Lecture, “ Fundamental Ideas and Problems of the Theory of Relativity .” (1921).

Millikan, R.A. “ A Direct Photoelectric Determination of Planck’s ‘h’ .”  Phys. Rev ., 7, 355 (1916).

Hughes, Arthur L., and Lee A. Du Bridge. Photoelectric Phenomena . Boston, MA: McGraw-Hill, (1932).

Discusses phenomena such as the velocity distribution of the electrons, effects of polarization and angle of incidence of the light, influence of the surface temperature, photoelectric behavior of thin films and composite materials, etc.

Harnwell, G. P., and Livingood, J. J. “Thermionic and Photoelectric Effects.” In Experimental Atomic Physics . Boston, MA: McGraw-Hill, 1933, pp. 214-223. ISBN: 9780070266605.

Melissinos, Adrian C. “Photoelectric Effect.” In Experiments in Modern Physics . New York, NY: Academic Press, (1968).

Selected Resources

Baumeister, P. and G. Pincus. “Optical Interference Coatings.”  Scientific American 223, 58-75 (December 1970).

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21.2 Einstein and the Photoelectric Effect

Section learning objectives.

By the end of this section, you will be able to do the following:

• Describe Einstein’s explanation of the photoelectric effect
• Describe how the photoelectric effect could not be explained by classical physics
• Calculate the energy of a photoelectron under given conditions
• Describe use of the photoelectric effect in biological applications, photoelectric devices and movie soundtracks

Teacher Support

The learning objectives in this section will help your students master the following standards:

• (D) : explain the impacts of the scientific contributions of a variety of historical and contemporary scientists on scientific thought and society.
• (A) : describe the photoelectric effect and the dual nature of light.

Section Key Terms

The Photoelectric Effect

[EL]Ask the students what they think the term photoelectric means. How does the term relate to its definition?

When light strikes certain materials, it can eject electrons from them. This is called the photoelectric effect , meaning that light ( photo ) produces electricity. One common use of the photoelectric effect is in light meters, such as those that adjust the automatic iris in various types of cameras. Another use is in solar cells, as you probably have in your calculator or have seen on a rooftop or a roadside sign. These make use of the photoelectric effect to convert light into electricity for running different devices.

[BL] [OL] Discuss with students what may cause light to eject electrons from a material. Are there certain materials that are more susceptible to having electrons ejected?

[AL] Ask students why a light meter would be useful in a camera. How could the number of electrons emitted from the light meter control the camera’s iris? Have students draw a diagram of the camera that may demonstrate this effect.

Revolutionary Properties of the Photoelectric Effect

When Max Planck theorized that energy was quantized in a blackbody radiator, it is unlikely that he would have recognized just how revolutionary his idea was. Using tools similar to the light meter in Figure 21.5 , it would take a scientist of Albert Einstein ’s stature to fully discover the implications of Max Planck’s radical concept.

Through careful observations of the photoelectric effect, Albert Einstein realized that there were several characteristics that could be explained only if EM radiation is itself quantized . While these characteristics will be explained a bit later in this section, you can already begin to appreciate why Einstein’s idea is very important. It means that the apparently continuous stream of energy in an EM wave is actually not a continuous stream at all. In fact, the EM wave itself is actually composed of tiny quantum packets of energy called photons .

In equation form, Einstein found the energy of a photon or photoelectron to be

where E is the energy of a photon of frequency f and h is Planck’s constant. A beam from a flashlight, which to this point had been considered a wave, instead could now be viewed as a series of photons, each providing a specific amount of energy see Figure 21.6 . Furthermore, the amount of energy within each individual photon is based upon its individual frequency, as dictated by E = h f . E = h f . As a result, the total amount of energy provided by the beam could now be viewed as the sum of all frequency-dependent photon energies added together.

It is important for students to be comfortable with the material to this point before moving forward. To ensure that they are, one task that you may have them do is to draw a few pictures similar to Figure 21.6 . Have the students draw photons leaving a low intensity flashlight vs. a high intensity flashlight, a high frequency flashlight vs. a low frequency flashlight, and a high wavelength flashlight vs. a low wavelength flashlight. These diagrams will help ensure the students understand fundamental concepts before moving to the difficult proofs that follow.

Just as with Planck’s blackbody radiation, Einstein’s concept of the photon could take hold in the scientific community only if it could succeed where classical physics failed. The photoelectric effect would be a key to demonstrating Einstein’s brilliance.

Consider the following five properties of the photoelectric effect. All of these properties are consistent with the idea that individual photons of EM radiation are absorbed by individual electrons in a material, with the electron gaining the photon’s energy. Some of these properties are inconsistent with the idea that EM radiation is a simple wave. For simplicity, let us consider what happens with monochromatic EM radiation in which all photons have the same energy hf .

• If we vary the frequency of the EM radiation falling on a clean metal surface, we find the following: For a given material, there is a threshold frequency f 0 for the EM radiation below which no electrons are ejected, regardless of intensity. Using the photon model, the explanation for this is clear. Individual photons interact with individual electrons. Thus if the energy of an individual photon is too low to break an electron away, no electrons will be ejected. However, if EM radiation were a simple wave, sufficient energy could be obtained simply by increasing the intensity.
• Once EM radiation falls on a material, electrons are ejected without delay . As soon as an individual photon of sufficiently high frequency is absorbed by an individual electron, the electron is ejected. If the EM radiation were a simple wave, several minutes would be required for sufficient energy to be deposited at the metal surface in order to eject an electron.
• The number of electrons ejected per unit time is proportional to the intensity of the EM radiation and to no other characteristic. High-intensity EM radiation consists of large numbers of photons per unit area, with all photons having the same characteristic energy, hf . The increased number of photons per unit area results in an increased number of electrons per unit area ejected.
• If we vary the intensity of the EM radiation and measure the energy of ejected electrons, we find the following: The maximum kinetic energy of ejected electrons is independent of the intensity of the EM radiation . Instead, as noted in point 3 above, increased intensity results in more electrons of the same energy being ejected. If EM radiation were a simple wave, a higher intensity could transfer more energy, and higher-energy electrons would be ejected.
• The kinetic energy KE of an ejected electron equals the photon energy minus the binding energy BE of the electron in the specific material. An individual photon can give all of its energy to an electron. The photon’s energy is partly used to break the electron away from the material. The remainder goes into the ejected electron’s kinetic energy. In equation form, this is given by

where K E e K E e is the maximum kinetic energy of the ejected electron, h f h f is the photon’s energy, and BE is the binding energy of the electron to the particular material. The binding energy is also often called the work function of the material. This equation explains the properties of the photoelectric effect quantitatively and demonstrates that BE is the minimum amount of energy necessary to eject an electron. If the energy supplied is less than BE, the electron cannot be ejected. The binding energy can also be written as B E = h f 0 , B E = h f 0 , where f 0 f 0 is the threshold frequency for the particular material. Figure 21.8 shows a graph of maximum K E e K E e versus the frequency of incident EM radiation falling on a particular material.

Show students Figure 21.8 . What would be the kinetic energy of an electron if f is less than f 0 ? What does this mean? Why would this be the case? These questions aim to help students internalize the concept of binding energy.

Tips For Success

The following five pieces of information can be difficult to follow without some organization. It may be useful to create a table of expected results of each of the five properties, with one column showing the classical wave model result and one column showing the modern photon model result.

The table may look something like Table 21.1

It may be useful to complete the table above as a class. This material takes some time to interpret, so encourage students to move slowly. Once completed, your table may look like Table 21.2 .

Virtual Physics

Photoelectric effect.

In this demonstration, see how light knocks electrons off a metal target, and recreate the experiment that spawned the field of quantum mechanics.

Grasp Check

In the circuit provided, what are the three ways to increase the current?

• increase the intensity, increase the wavelength, alter the target
• decrease the intensity, increase the wavelength, alter the target
• decrease the intensity, decrease the wavelength, alter the target
• increase the intensity, decrease the wavelength, alter the target

Worked Example

Photon energy and the photoelectric effect: a violet light.

(a) What is the energy in joules and electron volts of a photon of 420-nm violet light? (b) What is the maximum kinetic energy of electrons ejected from calcium by 420 nm violet light, given that the binding energy of electrons for calcium metal is 2.71 eV?

To solve part (a), note that the energy of a photon is given by E = h f E = h f . For part (b), once the energy of the photon is calculated, it is a straightforward application of K E e = h f − B E K E e = h f − B E to find the ejected electron’s maximum kinetic energy, since BE is given.

Photon energy is given by

E = h f . E = h f .

Since we are given the wavelength rather than the frequency, we solve the familiar relationship c = f λ c = f λ for the frequency, yielding

Combining these two equations gives the useful relationship

Now substituting known values yields

Converting to eV, the energy of the photon is

Finding the kinetic energy of the ejected electron is now a simple application of the equation K E e = h f − B E K E e = h f − B E . Substituting the photon energy and binding energy yields

The energy of this 420 nm photon of violet light is a tiny fraction of a joule, and so it is no wonder that a single photon would be difficult for us to sense directly—humans are more attuned to energies on the order of joules. But looking at the energy in electron volts, we can see that this photon has enough energy to affect atoms and molecules. A DNA molecule can be broken with about 1 eV of energy, for example, and typical atomic and molecular energies are on the order of eV, so that the photon in this example could have biological effects, such as sunburn. The ejected electron has rather low energy, and it would not travel far, except in a vacuum. The electron would be stopped by a retarding potential of only 0.26 eV, a slightly larger KE than calculated above. In fact, if the photon wavelength were longer and its energy less than 2.71 eV, then the formula would give a negative kinetic energy, an impossibility. This simply means that the 420 nm photons with their 2.96 eV energy are not much above the frequency threshold. You can see for yourself that the threshold wavelength is 458 nm (blue light). This means that if calcium metal were used in a light meter, the meter would be insensitive to wavelengths longer than those of blue light. Such a light meter would be completely insensitive to red light, for example.

Practice Problems

What is the longest-wavelength EM radiation that can eject a photoelectron from silver, given that the bonding energy is 4.73 eV ? Is this radiation in the visible range?

• 2.63 × 10 −7 m; No, the radiation is in microwave region.
• 2.63 × 10 −7 m; No, the radiation is in visible region.
• 2.63 × 10 −7 m; No, the radiation is in infrared region.
• 2.63 × 10 -7 m; No, the radiation is in ultraviolet region.

What is the maximum kinetic energy in eV of electrons ejected from sodium metal by 450-nm EM radiation, given that the binding energy is 2.28 eV?

Technological Applications of the Photoelectric Effect

While Einstein’s understanding of the photoelectric effect was a transformative discovery in the early 1900s, its presence is ubiquitous today. If you have watched streetlights turn on automatically in response to the setting sun, stopped elevator doors from closing simply by putting your hands between them, or turned on a water faucet by sliding your hands near it, you are familiar with the electric eye , a name given to a group of devices that use the photoelectric effect for detection.

All these devices rely on photoconductive cells. These cells are activated when light is absorbed by a semi-conductive material, knocking off a free electron. When this happens, an electron void is left behind, which attracts a nearby electron. The movement of this electron, and the resultant chain of electron movements, produces a current. If electron ejection continues, further holes are created, thereby increasing the electrical conductivity of the cell. This current can turn switches on and off and activate various familiar mechanisms.

One such mechanism takes place where you may not expect it. Next time you are at the movie theater, pay close attention to the sound coming out of the speakers. This sound is actually created using the photoelectric effect! The audiotape in the projector booth is a transparent piece of film of varying width. This film is fed between a photocell and a bright light produced by an exciter lamp. As the transparent portion of the film varies in width, the amount of light that strikes the photocell varies as well. As a result, the current in the photoconductive circuit changes with the width of the filmstrip. This changing current is converted to a changing frequency, which creates the soundtrack commonly heard in the theater.

Work In Physics

Solar energy physicist.

According to the U.S. Department of Energy, Earth receives enough sunlight each hour to power the entire globe for a year. While converting all of this energy is impossible, the job of the solar energy physicist is to explore and improve upon solar energy conversion technologies so that we may harness more of this abundant resource.

The field of solar energy is not a new one. For over half a century, satellites and spacecraft have utilized photovoltaic cells to create current and power their operations. As time has gone on, scientists have worked to adapt this process so that it may be used in homes, businesses, and full-scale power stations using solar cells like the one shown in Figure 21.9 .

Solar energy is converted to electrical energy in one of two manners: direct transfer through photovoltaic cells or thermal conversion through the use of a CSP, concentrating solar power, system. Unlike electric eyes, which trip a mechanism when current is lost, photovoltaic cells utilize semiconductors to directly transfer the electrons released through the photoelectric effect into a directed current. The energy from this current can then be converted for storage, or immediately used in an electric process. A CSP system is an indirect method of energy conversion. In this process, light from the Sun is channeled using parabolic mirrors. The light from these mirrors strikes a thermally conductive material, which then heats a pool of water. This water, in turn, is converted to steam, which turns a turbine and creates electricity. While indirect, this method has long been the traditional means of large-scale power generation.

There are, of course, limitations to the efficacy of solar power. Cloud cover, nightfall, and incident angle strike at high altitudes are all factors that directly influence the amount of light energy available. Additionally, the creation of photovoltaic cells requires rare-earth minerals that can be difficult to obtain. However, the major role of a solar energy physicist is to find ways to improve the efficiency of the solar energy conversion process. Currently, this is done by experimenting with new semi conductive materials, by refining current energy transfer methods, and by determining new ways of incorporating solar structures into the current power grid.

Additionally, many solar physicists are looking into ways to allow for increased solar use in impoverished, more remote locations. Because solar energy conversion does not require a connection to a large-scale power grid, research into thinner, more mobile materials will permit remote cultures to use solar cells to convert sunlight collected during the day into stored energy that can then be used at night.

Regardless of the application, solar energy physicists are an important part of the future in responsible energy growth. While a doctoral degree is often necessary for advanced research applications, a bachelor's or master's degree in a related science or engineering field is typically enough to gain access into the industry. Computer skills are very important for energy modeling, including knowledge of CAD software for design purposes. In addition, the ability to collaborate and communicate with others is critical to becoming a solar energy physicist.

What role does the photoelectric effect play in the research of a solar energy physicist?

• The understanding of photoelectric effect allows the physicist to understand the generation of light energy when using photovoltaic cells.
• The understanding of photoelectric effect allows the physicist to understand the generation of electrical energy when using photovoltaic cells.
• The understanding of photoelectric effect allows the physicist to understand the generation of electromagnetic energy when using photovoltaic cells.
• The understanding of photoelectric effect allows the physicist to understand the generation of magnetic energy when using photovoltaic cells.

Check Your Understanding

• A beam of light energy is now considered a continual stream of wave energy, not photons.
• A beam of light energy is now considered a collection of photons, each carrying its own individual energy.

True or false—Visible light is the only type of electromagnetic radiation that can cause the photoelectric effect.

• The photoelectric effect is a direct consequence of the particle nature of EM radiation.
• The photoelectric effect is a direct consequence of the wave nature of EM radiation.
• The photoelectric effect is a direct consequence of both the wave and particle nature of EM radiation.
• The photoelectric effect is a direct consequence of neither the wave nor the particle nature of EM radiation.

Which aspects of the photoelectric effect can only be explained using photons?

• aspects 1, 2, and 3
• aspects 1, 2, and 4
• aspects 1, 2, 4 and 5
• aspects 1, 2, 3, 4 and 5
• Solar energy transforms into electric energy.
• Solar energy transforms into mechanical energy.
• Solar energy transforms into thermal energy.
• In a photovoltaic cell, thermal energy transforms into electric energy.

True or false—A current is created in a photoconductive cell, even if only one electron is expelled from a photon strike.

• A photon is a quantum packet of energy; it has infinite mass.
• A photon is a quantum packet of energy; it is massless.
• A photon is a fundamental particle of an atom; it has infinite mass.
• A photon is a fundamental particle of an atom; it is massless.

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August 18, 2015

Einstein's Legacy: The Photoelectric Effect

Despite the popularity of Einstein's theories of relativity and his musings on black holes, Einstein's Nobel Prize in physics was actually awarded for his discovery of the photoelectric effect. This discovery revolutionized our understanding of the world around us. But what is the photoelectric effect?

By Everyday Einstein Sabrina Stierwalt

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Scientific American  presents  Everyday Einstein  by  Quick & Dirty Tips .  Scientific American  and Quick & Dirty Tips are both Macmillan companies.

When you think of Albert Einstein, what do you think of? General relativity? Black holes? Crazy hair? While he certainly made significant contributions to all of those topics during his lifetime, Albert Einstein was perhaps even more well known in his time for his work to understand the photoelectric effect. In fact, when he was awarded the Nobel Prize in Physics in 1921, the honor was stated to be “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."

This discovery is so important—and Nobel Prize worthy—because Einstein suggested for the first time that light is both a wave  and  a particle. This phenomenon, known as the wave-particle duality of light, is fundamental to all of quantum mechanics and has influenced the development of electron microscopes and solar cells.

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What Is the Photoelectric Effect? When light with energy above a certain threshold hits a metal surface, an electron that was previously bound to the metal is knocked loose. Each particle of light, called a photon, collides with an electron and uses some of its energy to dislodge it from the metal. The rest of the photon’s energy is transferred to the now free-roaming negative charge, called a photoelectron.

So why does this happen? What determines the energies (and speeds) of the emitted electrons? To understand the answers to these questions, we need to dig a little into the history of the discovery of the photoelectric effect.

>> Continue reading on QuickAndDirtyTips.com

• IIT JEE Study Material

Photoelectric Effect

The photoelectric effect is a phenomenon in which electrons are ejected from the surface of a metal when light is incident on it. These ejected electrons are called  photoelectrons . It is important to note that the emission of photoelectrons and the kinetic energy of the ejected photoelectrons is dependent on the frequency of the light that is incident on the metal’s surface. The process through which photoelectrons are ejected from the surface of the metal due to the action of light is commonly referred to as  photoemission .

Download Complete Chapter Notes of Structure of Atom Download Now

The photoelectric effect occurs because the electrons at the surface of the metal tend to absorb energy from the incident light and use it to overcome the attractive forces that bind them to the metallic nuclei. An illustration detailing the emission of photoelectrons as a result of the photoelectric effect is provided below.

History of the Photoelectric Effect Principle Formula Laws Governing the Photoelectric Effect Experimental Study of the Photoelectric Effect Einstien’s Photoelectric Equation Graphs Applications Solved Problems (Numericals)

Recommended Video

Photoelectric effect – basics.

Hertz and Lenard’s Observation

History of the Photoelectric Effect

The photoelectric effect was first introduced by Wilhelm Ludwig Franz Hallwachs in the year 1887, and the experimental verification was done by Heinrich Rudolf Hertz. They observed that when a surface is exposed to electromagnetic radiation at a higher threshold frequency, the radiation is absorbed, and the electrons are emitted. Today, we study the photoelectric effect as a phenomenon that involves a material absorbing electromagnetic radiation and releasing electrically charged particles.

To be more precise, light incident on the surface of a metal in the photoelectric effect causes electrons to be ejected. The electron ejected due to the photoelectric effect is called a photoelectron and is denoted by e – .  The current produced as a result of the ejected electrons is called photoelectric current.

Explaining the Photoelectric Effect: The Concept of Photons

The photoelectric effect cannot be explained by considering light as a wave. However, this phenomenon can be explained by the particle nature of light, in which light can be visualised as a stream of particles of electromagnetic energy. These ‘particles’ of light are called photons . The energy held by a photon is related to the frequency of the light via Planck’s equation .

E = h𝜈 = hc/λ

• E denotes the energy of the photon
• h is Planck’s constant
• 𝜈 denotes the frequency of the light
• c is the speed of light (in a vacuum)
• λ is the wavelength of the light

Thus, it can be understood that different frequencies of light carry photons of varying energies. For example, the frequency of blue light is greater than that of red light (the wavelength of blue light is much shorter than the wavelength of red light). Therefore, the energy held by a photon of blue light will be greater than the energy held by a photon of red light.

Threshold Energy for the Photoelectric Effect

For the photoelectric effect to occur, the photons that are incident on the surface of the metal must carry sufficient energy to overcome the attractive forces that bind the electrons to the nuclei of the metals. The minimum amount of energy required to remove an electron from the metal is called the  threshold energy  (denoted by the symbol Φ). For a photon to possess energy equal to the threshold energy, its frequency must be equal to the  threshold frequency  (which is the minimum frequency of light required for the photoelectric effect to occur). The threshold frequency is usually denoted by the symbol 𝜈 th , and the associated wavelength (called the threshold wavelength) is denoted by the symbol λ th . The relationship between the threshold energy and the threshold frequency can be expressed as follows.

Φ = h𝜈 th  = hc/λ th

Relationship between the Frequency of the Incident Photon and the Kinetic Energy of the Emitted Photoelectron

Therefore, the relationship between the energy of the photon and the kinetic energy of the emitted photoelectron can be written as follows:

E photon  = Φ + E electron

⇒  h𝜈 = h𝜈 th  + ½m e v 2

• E photon  denotes the energy of the incident photon, which is equal to h𝜈
• Φ denotes the threshold energy of the metal surface, which is equal to h𝜈 th
• E electron  denotes the kinetic energy of the photoelectron, which is equal to ½m e v 2  (m e = Mass of electron = 9.1*10 -31  kg)

If the energy of the photon is less than the threshold energy, there will be no emission of photoelectrons (since the attractive forces between the nuclei and the electrons cannot be overcome). Thus, the photoelectric effect will not occur if 𝜈 < 𝜈 th . If the frequency of the photon is exactly equal to the threshold frequency (𝜈 = 𝜈 th ), there will be an emission of photoelectrons, but their kinetic energy will be equal to zero. An illustration detailing the effect of the frequency of the incident light on the kinetic energy of the photoelectron is provided below.

From the image, it can be observed that

• The photoelectric effect does not occur when the red light strikes the metallic surface because the frequency of red light is lower than the threshold frequency of the metal.
• The photoelectric effect occurs when green light strikes the metallic surface, and photoelectrons are emitted.
• The photoelectric effect also occurs when blue light strikes the metallic surface. However, the kinetic energies of the emitted photoelectrons are much higher for blue light than for green light. This is because blue light has a greater frequency than green light.

It is important to note that the threshold energy varies from metal to metal. This is because the attractive forces that bind the electrons to the metal are different for different metals. It can also be noted that the photoelectric effect can also take place in non-metals, but the threshold frequencies of non-metallic substances are usually very high.

Einstein’s Contributions towards the Photoelectric Effect

The photoelectric effect is the process that involves the ejection or release of electrons from the surface of materials (generally a metal) when light falls on them. The photoelectric effect is an important concept that enables us to clearly understand the quantum nature of light and electrons.

After continuous research in this field, the explanation for the photoelectric effect was successfully explained by Albert Einstein. He concluded that this effect occurred as a result of light energy being carried in discrete quantised packets. For this excellent work, he was honoured with the Nobel Prize in 1921.

According to Einstein, each photon of energy E is

Where E = Energy of the photon in joule

h = Plank’s constant (6.626 × 10 -34 J.s)

ν = Frequency of photon in Hz

Properties of the Photon

• For a photon, all the quantum numbers are zero.
• A photon does not have any mass or charge, and they are not reflected in a magnetic and electric field.
• The photon moves at the speed of light in empty space.
• During the interaction of matter with radiation, radiation behaves as it is made up of small particles called photons.
• Photons are virtual particles. The photon energy is directly proportional to its frequency and inversely proportional to its wavelength.
• The momentum and energy of the photons are related, as given below

E = p.c where

p = Magnitude of the momentum

c = Speed of light

Definition of the Photoelectric Effect

Principle of the photoelectric effect.

The law of conservation of energy forms the basis for the photoelectric effect.

Minimum Condition for Photoelectric Effect

Threshold frequency (γ th ).

It is the minimum frequency of the incident light or radiation that will produce a photoelectric effect, i.e., the ejection of photoelectrons from a metal surface is known as the threshold frequency for the metal. It is constant for a specific metal but may be different for different metals.

If γ = Frequency of the incident photon and γ th = Threshold frequency, then,

• If γ < γ Th , there will be no ejection of photoelectron and, therefore, no photoelectric effect.
• If γ = γ Th , photoelectrons are just ejected from the metal surface; in this case, the kinetic energy of the electron is zero.
• If γ > γ Th , then photoelectrons will come out of the surface, along with kinetic energy.

Threshold Wavelength (λ th )

During the emission of electrons, a metal surface corresponding to the greatest wavelength to incident light is known as threshold wavelength.

λ th  = c/γ th

For wavelengths above this threshold, there will be no photoelectron emission. For λ = wavelength of the incident photon, then

• If λ < λ Th , then the photoelectric effect will take place, and ejected electron will possess kinetic energy.
• If λ = λ Th, then just the photoelectric effect will take place, and the kinetic energy of ejected photoelectron will be zero.
• If λ > λ Th, there will be no photoelectric effect.

Work Function or Threshold Energy (Φ)

The minimal energy of thermodynamic work that is needed to remove an electron from a conductor to a point in the vacuum immediately outside the surface of the conductor is known as work function/threshold energy.

Φ = hγ th  = hc/λ th

The work function is the characteristic of a given metal. If E = energy of an incident photon, then

• If E < Φ, no photoelectric effect will take place.
• If E = Φ, just a photoelectric effect will take place, but the kinetic energy of ejected photoelectron will be zero
• If E > photoelectron will be zero
• If E > Φ, the photoelectric effect will take place along with the possession of the kinetic energy by the ejected electron.

Photoelectric Effect Formula

According to  Einstein’s explanation of the photoelectric effect ,

The energy of photon = Energy needed to remove an electron + Kinetic energy of the emitted electron

i.e., hν = W + E

• ν is the frequency of the incident photon
• W is a work function
• E is the maximum kinetic energy of ejected electrons: 1/2 mv²

Laws Governing the Photoelectric Effect

• For a light of any given frequency,; (γ > γ Th ), the photoelectric current is directly proportional to the intensity of light.
• For any given material, there is a certain minimum (energy) frequency, called threshold frequency, below which the emission of photoelectrons stops completely, no matter how high the intensity of incident light is.
• The maximum kinetic energy of the photoelectrons is found to increase with the increase in the frequency of incident light, provided the frequency (γ > γ Th ) exceeds the threshold limit. The maximum kinetic energy is independent of the intensity of light.
• The photo-emission is an instantaneous process.

Experimental Study of the Photoelectric Effect

Photoelectric Effect: Experimental Setup

The given experiment is used to study the photoelectric effect experimentally. In an evacuated glass tube, two zinc plates, C and D, are enclosed. Plates C acts as an anode, and D acts as a photosensitive plate.

Two plates are connected to battery B and ammeter A. If the radiation is incident on plate D through a quartz window, W electrons are ejected out of the plate, and current flows in the circuit. This is known as photocurrent. Plate C can be maintained at desired potential (+ve or – ve) with respect to plate D.

Characteristics of the Photoelectric Effect

• The threshold frequency varies with the material, it is different for different materials.
• The photoelectric current is directly proportional to the light intensity.
• The kinetic energy of the photoelectrons is directly proportional to the light frequency.
• The stopping potential is directly proportional to the frequency, and the process is instantaneous.

Factors Affecting the Photoelectric Effect

With the help of this apparatus, we will now study the dependence of the photoelectric effect on the following factors:

• The intensity of incident radiation.
• A potential difference between the metal plate and collector.
• Frequency of incident radiation.

Effects of Intensity of Incident Radiation on Photoelectric Effect

The potential difference between the metal plate, collector and frequency of incident light is kept constant, and the intensity of light is varied.

The electrode C, i.e., the collecting electrode, is made positive with respect to D (metal plate). For a fixed value of frequency and the potential between the metal plate and collector, the photoelectric current is noted in accordance with the intensity of incident radiation.

It shows that photoelectric current and intensity of incident radiation both are proportional to each other. The photoelectric current gives an account of the number of photoelectrons ejected per sec.

Effects of Potential Difference between the Metal Plate and the Collector on the Photoelectric Effect

The frequency of incident light and intensity is kept constant, and the potential difference between the plates is varied.

Keeping the intensity and frequency of light constant, the positive potential of C is increased gradually. Photoelectric current increases when there is a positive increase in the potential between the metal plate and the collector up to a characteristic value.

There is no change in photoelectric current when the potential is increased higher than the characteristic value for any increase in the accelerating voltage. This maximum value of the current is called saturation current.

Effect of Frequency on Photoelectric Effect

The intensity of light is kept constant, and the frequency of light is varied.

For a fixed intensity of incident light, variation in the frequency of incident light produces a linear variation of the cut-off potential/stopping potential of the metal. It is shown that the cut-off potential (Vc) is linearly proportional to the frequency of incident light.

The kinetic energy of the photoelectrons increases directly proportionally to the frequency of incident light to completely stop the photoelectrons. We should reverse and increase the potential between the metal plate and collector in (negative value) so the emitted photoelectron can’t reach the collector.

Einstein’s Photoelectric Equation

According to Einstein’s theory of the photoelectric effect, when a photon collides inelastically with electrons, the photon is absorbed completely or partially by the electrons. So if an electron in a metal absorbs a photon of energy, it uses the energy in the following ways.

Some energy Φ 0  is used in making the surface electron free from the metal. It is known as the work function of the material. Rest energy will appear as kinetic energy (K) of the emitted photoelectrons.

Einstein’s Photoelectric Equation Explains the Following Concepts

• The frequency of the incident light is directly proportional to the kinetic energy of the electrons, and the wavelengths of incident light are inversely proportional to the kinetic energy of the electrons.
• If γ = γ th or λ =λ th then v max = 0
• γ < γ th  or λ > λ th : There will be no emission of photoelectrons.
• The intensity of the radiation or incident light refers to the number of photons in the light beam. More intensity means more photons and vice-versa. Intensity has nothing to do with the energy of the photon. Therefore, the intensity of the radiation is increased, and the rate of emission increases, but there will be no change in the kinetic energy of electrons. With an increasing number of emitted electrons, the value of the photoelectric current increases.

Different Graphs of the Photoelectric Equation

• Photoelectric current vs Retarding potential for different voltages
• Photoelectric current vs Retarding potential for different intensities
• Electron current vs Light Intensity
• Stopping potential vs Frequency
• Electron current vs Light frequency
• Electron kinetic energy vs Light frequency

Applications of the Photoelectric Effect

• Used to generate electricity in solar panels. These panels contain metal combinations that allow electricity generation from a wide range of wavelengths.
• Motion and Position Sensors: In this case, a photoelectric material is placed in front of a UV or IR LED. When an object is placed in between the Light-emitting diode (LED) and sensor, light is cut off, and the electronic circuit registers a change in potential difference
• Lighting sensors, such as the ones used in smartphones, enable automatic adjustment of screen brightness according to the lighting. This is because the amount of current generated via the photoelectric effect is dependent on the intensity of light hitting the sensor.
• Digital cameras can detect and record light because they have photoelectric sensors that respond to different colours of light.
• X-Ray Photoelectron Spectroscopy (XPS): This technique uses X-rays to irradiate a surface and measure the kinetic energies of the emitted electrons. Important aspects of the chemistry of a surface can be obtained, such as elemental composition, chemical composition, the empirical formula of compounds and chemical state.
• Photoelectric cells are used in burglar alarms.
• Used in photomultipliers to detect low levels of light.
• Used in video camera tubes in the early days of television.
• Night vision devices are based on this effect.
• The photoelectric effect also contributes to the study of certain nuclear processes. It takes part in the chemical analysis of materials since emitted electrons tend to carry specific energy that is characteristic of the atomic source.

Photoelectric Effect – JEE Advanced Concepts and Problems

Problems on the Photoelectric Effect

1. In a photoelectric effect experiment, the threshold wavelength of incident light is 260 nm and E (in eV) = 1237/λ (nm). Find the maximum kinetic energy of emitted electrons.

⇒ K max  = (1237) × [(380 – 260)/380×260] = 1.5 eV

Therefore, the maximum kinetic energy of emitted electrons in the photoelectric effect is 1.5 eV.

2. In a photoelectric experiment, the wavelength of the light incident on metal is changed from 300 nm to 400 nm and (hc/e = 1240 nm-V). Find the decrease in the stopping potential.

hc/λ 1  = ϕ + eV 1 . . . . (i)

hc/λ 2  = ϕ + eV 2 . . . . (ii)

Equation (i) – (ii)

hc(1/λ 1 – 1/λ 2 ) = e × (V 1 – V 2 )

= (1240 nm V) × 100nm/(300nm × 400nm)

=12.4/12 ≈ 1V

Therefore, the decrease in the stopping potential during the photoelectric experiment is 1V.

3. When ultraviolet light with a wavelength of 230 nm shines on a particular metal plate, electrons are emitted from plate 1, crossing the gap to plate 2 and causing a current to flow through the wire connecting the two plates. The battery voltage is gradually increased until the current in the ammeter drops to zero, at which point the battery voltage is 1.30 V. 

a) What is the energy of the photons in the beam of light in eV?

b) What is the maximum kinetic energy of the emitted electrons in eV?

Assuming that the wavelength corresponds to the wavelength in the vacuum.

f = 1.25 × 10 15 Hz

The energy of photon E = hf

E = (4.136 × 10 -15 )( 1.25 × 10 15)   

Note: Planck’s constant in eV s = 4.136 × 10 -15  eV s

E = 5.17 eV.

b) The maximum kinetic energy related to the emitted electron is stopping potential. In this case, the stopping potential is 1.30V. So the maximum kinetic energy of the electrons is 1.30V.

Also Check out:  JEE Main Photoelectric Effect Previous Year Questions with Solutions

Important Points to Remember

• If we consider the light with any given frequency, the photoelectric current is generally directly proportional to the intensity of light. However, the frequency should be above the threshold frequency in such a case.
• Below threshold frequency, the emission of photoelectrons completely stops despite the high intensity of incident light.
• A photoelectron’s maximum kinetic energy increases with an increase in the frequency of incident light. In this case, the frequency should exceed the threshold limit. Maximum kinetic energy is not affected by the intensity of light.
• Stopping potential is the negative potential of the opposite electrode when the photo-electric current falls to zero.
• The threshold frequency is described as the frequency when the photoelectric current stops below a particular frequency of incident light.
• The photoelectric effect establishes the quantum nature of radiation. This has been taken into account to be proof in favour of the particle nature of light.

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Anti- Toxoplasma gondii effects of XYP1-derived peptides and regulatory mechanisms of XYP1

• Jing Li 1   na1 ,
• Kaijuan Wu 1   na1 ,
• Xiaohua Liu 1 ,
• Dongqian Yang 1 ,
• Jing Xie 1 ,
• Yixiao Wang 1 ,
• Kang Liu 1 ,
• Zheng Wang 3 ,
• Wei Liu 4 &
• Liping Jiang 1 , 2  

Parasites & Vectors volume  17 , Article number:  376 ( 2024 ) Cite this article

Metrics details

Toxoplasmosis, caused by Toxoplasma gondii , poses serious health issues for humans and animals. Individuals with impaired immune systems are more susceptible to severe toxoplasmosis. Pregnant women infected by T. gondii can face the possibility of birth defects and miscarriages. While pyrimethamine and sulfadiazine are commonly used drugs in clinical practice, concerns over their side effects and resistance are on the rise. A spider peptide XYP1 isolated from Lycosa coelestis had potent anti- T. gondii effects, but it had a high synthesis cost and strong cytotoxicity.

This study intended to modify XYP1 for producing derived peptides via amino acid truncation and substitution. The anti- T. gondii effect was evaluated by trypan blue staining assay and killing experiment of RH strain tachyzoites. The CCK8 and hemolysis assays were used to compare their safeties. The morphological changes of T. gondii were observed by scanning electron microscope and transmission electron microscope. In addition, the mechanism of XYP1 against T. gondii through RNA-sequencing was further explored.

In vivo and in vitro experiments revealed that XYP1-18 and XYP1-18-1 had excellent anti- T. gondii activity with lower cytotoxicity and hemolysis activity than XYP1. XYP1, XYP1-18, and XYP1-18-1 were able to disrupt the surface membrane integrity of T. gondii tachyzoites, forming pores and causing the disruption of organelles. Furthermore, RNA-sequencing analysis indicated that XYP1 could stimulate the host immune response to effectively eliminate T. gondii and lessen the host’s inflammatory reaction.

Conclusions

XYP1-18 had lower cytotoxicity and hemolysis activity than XYP1, as well as significantly extending the survival time of the mice. XYP1 played a role in host inflammation and immune responses, revealing its potential mechanism. Our research provided valuable insights into the development and application of peptide-based drugs, offering novel strategies and directions for treating toxoplasmosis.

Graphical Abstract

Toxoplasma gondii ( T. gondii ) is an important opportunistic pathogen belonging to the phylum Apicomplexa. T. gondii can cause toxoplasmosis, which is a zoonotic parasitic disease affecting both humans and animals [ 1 ]. Globally, approximately one-third of the population is estimated to be infected with T. gondii [ 1 , 2 ]. In immunocompromised patients, T. gondii infection can lead to significant morbidity and mortality. Currently, drugs such as pyrimethamine combined with sulfadiazine, trimethoprim combined with sulfamethoxazole, and spiramycin are mainly used in clinical treatment of toxoplasmosis [ 3 , 4 ]. While these treatments can alleviate symptoms and control the disease, the limitations and side effects of drug treatment need to be taken into account [ 5 ]. T. gondii also showed increasing resistance to sulfadiazine and pyrimethamine [ 6 ]. Therefore, it is crucial to explore new treatment methods and drugs for the effective management of toxoplasmosis.

In recent years, peptide drugs have emerged as a novel therapeutic approach with various applications such as antiinflammatory, antitumor, and antimicrobial properties, making them valuable in treating a wide range of diseases [ 7 , 8 , 9 ]. Peptides offer the advantage of specific targeting toward diseases while reducing the complexity of toxic side effects and drug metabolism, thus becoming a new focus in drug research for treating multiple diseases. Studies have identified peptides with anti- T. gondii activity. For instance, cal14.1a was a peptide extracted from Conus californicus , which can reduce the invasion and proliferation of T. gondii in host cells [ 10 ]. Additionally, longicin P4, an alkaline peptide extracted from Haemaphysalis longicornis , had shown efficacy in limiting the growth of T. gondii [ 11 ]. HPRP-A1 and HPRP-A2, α-helical cationic peptides extracted from Helicobacter pylori , can effectively reduce the survival rate of T. gondii tachyzoites and inhibit their adhesion and invasion of macrophages [ 12 ]. These indicated that peptides still hold great potential and application space in the fight against T. gondii infection.

Our earlier studies have indicated that a novel spider polypeptide XYP1 derived from the venom of Lycosa coelestis suppressed the invasion and proliferation of T. gondii and extended the survival time of mice infected with T. gondii [ 13 ]. Compared with the conventional drug sulfadiazine, the peptide XYP1 showed superior therapeutic efficacy and fewer side effects, thus holding significant promise for the treatment of T. gondii infection. However, limitations such as strong cytotoxicity and high synthesis cost restricted its clinical application.

Peptide truncation can not only save the synthesis cost, reduce reaction time, and enhance efficiency, but also can alter the bioactivity, stability, hydrophilicity, hydrophobicity, and toxicity of peptides. The beneficial effects of peptide truncation have been reflected in arasin 1 and linear chicken β-defensin-4 [ 14 , 15 , 16 ]. The distribution of hydrophobic and hydrophilic residues is a crucial factor influencing the activity of antimicrobial peptides (AMPs), with hydrophilic residues typically concentrated near the membrane surface of AMPs, while hydrophobic residues are distributed internally. This distribution pattern facilitates the interaction of AMPs with the cell membrane, so as to enhance their bactericidal activity [ 17 , 18 ].

In this study, XYP1 was truncated and replaced with amino acids to produce eight derived peptides. Two peptides, XYP1-18 and XYP1-18-1, with high anti- T. gondii activity were selected. XYP1-18 and XYP1-18-1 exhibited lower cytotoxicity and hemolysis compared with XYP1. Further in vivo and in vitro experiments indicated that XYP1-18 had similar activity as XYP1. The three peptides can damage the surface membrane and internal structure of T. gondii tachyzoites to different degrees. Additionally, we discovered that XYP1 can protect the host by reducing the inflammatory response and inducing immune response during T. gondii infection. In conclusion, our strategy may provide more insights and possibilities for peptide drug design and development, offering a novel approach for utilizing peptides to combat T. gondii infection in the future.

Parasites and cell culture

Tachyzoites of the RH-GFP-TgAtg8 and RH wild-type T. gondii strains were transmitted in human foreskin fibroblasts (HFFs). HFFs were growth in Dulbecco’s modified Eagle’s medium (DMEM, Gibco, USA) attached with 1% antibiotics (10 mg/mL streptomycin solution, 25 μg/mL amphotericin B, and 10,000 U/mL penicillin) (Sangon Biotech, China) and 10% fetal bovine serum (FBS, Invitrogen, USA) at 37 ℃ with the concentration of 5% CO 2. The different T. gondii strains were maintained in HFFs with DMEM, 2% FBS, and 1% antibiotics in the same environment [ 19 ].

The 6–8-week-old female BALB/c and Kunming mice used in this study were purchased from the Department of Laboratory Animals, Central South University in China.

Sequence and structure analysis of polypeptides

The various characteristics of polypeptides including isoelectric point, net charge, and average hydrophilicity were analyzed on the website ( https://www.expasy.org/ ) [ 20 ]. The tertiary structure of polypeptides was predicted by I-TASSER's online tool ( http://zhanglab.ccmb.med.umich.edu/I-TASSER/ ) [ 21 ]. The spiral round figure was constructed by Heliquest online website ( http://heliquest.ipmc.cnrs.fr/ ) [ 22 ].

Chemical synthesis and identification of XYP1-derived peptides

The derived peptides were synthesized by Fmoc solid phase polypeptide synthesis method [ 23 ]. It mainly included the following steps: deprotection of Fmoc group, coupling reaction, washing, detection using the Kaiser method, and cleavage. High performance liquid chromatography (HPLC) technique (Shimadzu, Japan) was used to purify peptides, and electrospray ionization mass spectrometry (ESI–MS) (Shimadzu, Japan) was used to identification of derived peptides.

Evaluation of anti -toxoplasma gondii effect

When T. gondii died, trypan blue was able to cross its cell membrane and enter the interior, making it blue. Trypan blue exclusion test was undertaken as mentioned before [ 24 ]. Firstly, 10 μM different XYP1-derived peptides were prepared, incubated with T. gondii for 2 h, and then the trypan blue dye was added. After 3–5 min, the survival rate of T. gondii was observed by optical microscope and the mortality rate was calculated.

The tachyzoites of RH-GFP-TgAtg8 strain did not fluorescein after death, which can be used to determine the difference in the anti- T. gondii activity of XYP1 and its derived peptides; 10 μM different derived peptides were incubated with tachyzoites of RH-GFP-TgAtg8 strain, respectively, and the fluorescence intensity and area were observed under fluorescence microscope (Leica DM IL LED, Germany).

Hemolysis and cytotoxicity assays

The hemolytic activity of peptides was determined by the amount of hemoglobin released from lysed human red blood cells [ 25 ]. Normal human blood was mixed 1:1 with Aldrin’s fluid, followed by centrifuging, and phosphate buffered saline (PBS) was added to make 1% red blood cell resuspension. The 50 μL resuspension was mixed with 50 μL PBS, 1% Triton X-100, 70% DMSO (Sigma, USA), and different concentrations of polypeptides, then incubated at 37 ℃ for 30 min. After centrifugation, the OD 540 value of the supernatant was measured by Microplate reader (Agilent, USA) and the hemolysis rate was calculated.

The CCK-8 method [ 26 ] was employed to assess the effect of different peptides on cytotoxicity, and 100 μL of cell culture solution containing 10 μM different peptides were added to each of the 96-well plates. After 24 h, 10 μL CCK-8 solution (APExBIO, USA) was added and incubated for another 2 h. The OD 540 value was then measured and the cell mortality was calculated.

Survival assay

The anti- T. gondii effect of peptides in vivo was investigated through a mouse survival experiment [ 27 ]. The experiment was divided into a PBS control group and different polypeptide groups. Each group of mice was intraperitoneally injected with 1000 RH strain tachyzoites. After 4 h, the pre-prepared 4 mg/mL peptides were injected into the mice. The survival time of mice in each group was recorded and analyzed statistically.

Electron microscopy technology

The scanning electron microscope (SEM) was used to observe the surface structure of T. gondii , while the transmission electron microscope (TEM) can reflect the morphological changes of T. gondii inside the cell [ 28 ]. The sample preparation process for SEM included: the preparation process of SEM consists of fixing the T. gondii sample with 4% glutaraldehyde solution and staying overnight at 4 ℃. A series of gradient dehydration treatments are performed, followed by dehydration in anhydrous ethanol. After dehydration, the tachyzoites were dried and coated, followed by SEM observation (Hitachi S-3400N, Japan).

TEM steps mainly include fixing, rinsing, dehydration, infiltration, embedding, etc. After the T. gondii samples were fixed in 4% glutaraldehyde, they were washed with a cacodylate buffer, and 1% osmium tetroxide was used to fix the sample at 4 ℃ for 2 h. The fixed samples were then washed and subjected to a series of gradient dewatering treatments before being embedded in epoxy resin and polymerized at 60 ℃. After polymerization, the sample was sliced and double stained. Finally, the internal structure of T. gondii tachyzoites treated with different polypeptides was observed by TEM (Tecnai G2 Spirit TWIN, USA).

RNA-seq analysis

The control group consisted of HFF cells infected with T. gondii without XYP1, and the experimental group was treated with XYP1 for 8 h. Total RNAs were extracted from the control and experimental groups and reverse to cDNA, with three replicates per group. After the steps of end repair, A-tailing, and adapter ligation, the samples were proceeded with polymerase chain reaction (PCR) amplification and Illumina transcriptome sequencing in the OE biotech Co., Ltd. (Shanghai, China). Trimmomatic software [ 29 ] was used to filter the raw data to obtain high-quality data information. Subsequently, the filtered data was aligned and assembled using HISAT2 [ 30 ] and StringTie [ 31 ] tool. The number of reads mapped to each gene in each sample was calculated.

DESeq software [ 31 ] was used to screen differentially expressed genes, and the screening conditions for differential genes were fold change > 2 and P value < 0.05. To further understand the function of differentially expressed genes, cluster analysis, gene ontology (GO) functional enrichment analysis [ 32 ] and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis [ 33 ] were conducted on the selected genes. Additionally, the PPI network of differentially expressed proteins was constructed by the STRING website, and hub genes were identified by Cytoscape software.

Quantitative real-time PCR (qRT-PCR)

The total RNAs from different samples were extracted using the classical Trizol method. The PerfectStart Uni RT&qPCR Kit (TransGen Biotech, China) was used to efficiently reverse transcribe RNA into cDNA and synthesize the first strand. The qRT-PCR reaction system contains cDNA temple, forward and reverse primers, double stranded DNA fluorescent dye (SYBR Green I), DNA polymerase, dNTP and buffer. The PCR reaction was performed using a two-step amplification procedure: 94 ℃ for 30 s (pre-denaturation) followed by 40 cycles of 94 ℃ for 5 s and 60 ℃ for 30 s. Finally, the relative expression of each gene was calculated by 2 −∆∆Cq method.

Statistical analysis

The quantitative data was analyzed with the method of two-tailed Student’s t -test or two-way analysis of variance (ANOVA) with Tukey’s multiple comparisons in GraphPad 8.4 software. A P -value < 0.05 was considered to have a significant difference.

Sequence analysis and structural modification of XYP1

XYP1, as a novel anti- T. gondii polypeptide, showed good anti- T. gondii activity [ 13 ]. To further reduce the cytotoxicity and synthetic cost of XYP1, bioinformatic analyses and structural modifications were performed to optimize the structure and function of XYP1. On the basis of the predicted tertiary structure of XYP1 (KIKWFKAMKSIAKFIAKDQLKKHL, Fig.  1 A), the α-helix structure of the N-terminal was retained and truncated from the C-terminal, resulting in four truncated peptides: XYP1-15, XYP1-16, XYP1-17, and XYP1-18 (Fig.  1 ). The structure with hydrophobic and hydrophilic residues distributed on both sides is beneficial for enhancing the activity of antimicrobial peptides (AMPs). Therefore, according to the helical wheel projection diagrams of the truncated peptides, the amino acid substitutions were performed on the four peptides. Specifically, leucine (Leu, L) instead of lysine (Lys, K), K instead of isoleucine (Ile, I), K instead of alanine (Ala, A), serine (Ser, S) was replaced with K, phenylalanine (Phe, F) was replaced with lysine (K), and aspartic acid (Asp, D) was replaced with leucine (L). As shown in Fig.  1 , four peptides were eventually generated: XYP1-15-1 (LKKWFKKMKKIAKKI), XYP1-16-1 (LKKWFKKMKKIAKKIA), XYP1-17-1 (LKKWFKKMKKIAKKIAK), and XYP1-18-1 (LKKWFKKMKKIAKKIAKL). The properties and parameters of XYP1 and its derived peptides were shown in Additional file 1 : Table S1. The high-purity polypeptides were successfully synthesized by solid-phase synthesis. The results of ESI–MS (Additional file 1 : Figs. S1 and S2) showed that the actual molecular weight of each polypeptide was consistent with the theoretical molecular weight.

Structure diagram of XYP1 and its derived peptides. A The predicted tertiary structure diagram of XYP1, the dotted red boxes are the amino acids intended to be truncated. B The whole diagram is divided into four small pictures, with the left being the structure diagram and spiral wheel diagram of XYP1-15, and the right being the schematic diagram of XYP1-15-1; the residues after amino acid replacement of XYP1-15 are marked in blue. C The left diagram is the structure diagram and spiral wheel diagram of XYP1-16, and the right diagram exhibits XYP1-16-1; the residues after amino acid replacement of XYP1-16 are marked in blue. D The diagram on the left describes the structure and properties of XYP1-17, and the right diagram represents XYP1-17-1; blue marks the amino acids that have been replaced. E The figures on the left and right show the structure and properties of the derived peptides XYP1-18 and XYP1-18-1, respectively

The derived peptides XYP1-17, XYP1-18, and XYP1-18-1 exhibited anti- T. gondii activity

Our previous study has confirmed that 10 μM XYP1 can kill T. gondii tachyzoites. Therefore, trypan blue staining assay was used to compare the killing activity in vitro of 10 μM different derived peptides and XYP1. The result of XYP1 was consistent with the previous findings. Among these derived peptides, XYP1-17, XYP1-18, and XYP1-18-1 also exhibited good anti- T. gondii activity, with XYP1-18 being comparable to XYP1 (Fig.  2 A). To further demonstrate the anti- T. gondii activity of XYP1-17, XYP1-17-1, XYP1-18, and XYP1-18-1, the tachyzoites of RH-GFP-TgAtg8 T. gondii strain were co-incubated with 10 μM concentrations of these peptides. After a 2-h incubation, we observed their anti- T. gondii activity through fluorescence microscopy (Fig.  2 B). The fluorescence area and intensity of XYP1-17, XYP1-18, and XYP1-18-1 groups were significantly reduced compared with the control group (Fig.  2 C, D ). These results were consistent with the trypan blue staining assay.

Inhibitory effect of XYP1 and its derived peptides on T. gondii in vitro. A Statistical diagram of toxoplasma mortality in each group in trypan blue experiment, the killing effect of PBS (negative control group), DMSO (negative control group), and XYP1 and its derived peptides (experimental group) on T. gondii at a concentration of 10 μM. * P  < 0.05, ** P  < 0.01, ## P  < 0.01, *** P  < 0.001, **** P  < 0.0001, “ns” shows P  > 0.05. B Fluorescence microscopy results of control group and XYP1 and derived peptides (XYP1-17, XYP1-18, XYP1-18-1), scale = 100 μm. C–D Statistical analysis of fluorescence area and fluorescence intensity of each group, * P  < 0.05, ** P  < 0.01, *** P  < 0.001, **** P  < 0.0001, “ns” shows P  > 0.05

To explore the concentration dependence of the polypeptides against T. gondii , XYP1-derived peptide solutions with different concentration gradients (5 μM, 10 μM, 20 μM) were prepared and then mixed with T. gondii tachyzoites at room temperature. After incubation for 2 h, the anti- T. gondii effect was observed by fluorescence microscopy. The results showed that the killing effect of these three peptides on tachyzoites of RH-GFP-TgAtg8 strain was enhanced with increasing concentration (Fig.  3 A), and the fluorescence density of T. gondii became weaker (Fig.  3 B). Compared with the control group, the fluorescence intensity and fluorescence area of XYP1 and XYP1-18 at different concentrations were statistically different.

Safety assessment and survival experiments of XYP1-derived peptides. A , B The killing effect of derived peptides on T. gondii at different concentrations; Fig. A is the statistical analysis of fluorescence area of each group, Fig. B is the statistical analysis of fluorescence intensity of each group. * P  < 0.05, ** P  < 0.01, *** P  < 0.001, **** P  < 0.0001, “ns” shows P  > 0.05. C Cytotoxic effects of XYP1, XYP1-18, and XYP1-18-1 at different concentrations on HFFs; each graph shows the IC 50 value of each polypeptide to the HFFs. D Hemolytic activity of XYP1, XYP1-18, and XYP1-18-1 polypeptides on human red blood cells, hemolysis rate is a measure of hemolytic activity. E Survival of mice infected with T. gondii after treatment with three peptides: XYP1, XYP1-18, and XYP1-18-1, all drugs were administered to mice at a dose of 4 mg/kg, * P  < 0.05, ** P  < 0.01, *** P  < 0.001, **** P  < 0.0001, “ns” shows P  > 0.05

The security of XYP1-18 and XYP1-18-1 was higher than XYP1

According to the results of trypan blue staining and fluorescence microscopy, the peptides XYP1-18 and XYP1-18-1 had better anti- T. gondii effects. Therefore, these two peptides were selected for the cytotoxicity test. It was observed that the higher the concentration of XYP1, XYP1-18, and XYP1-18-1, the greater the toxicity to cells (Fig.  3 C). The IC 50 values of XYP1, XYP1-18, and XYP1-18-1 were 33.63 μM, 80.60 μM, and 45.60 μM, respectively. The IC 50 values of XYP1-18 and XYP1-18-1 were higher than XYP1, indicating higher safety of these two derived peptides.

Hemolysis assay was also used to evaluate the toxic effects of peptides on the host. When XYP1 concentrations ranged from 1.25 μM to 10 μM, almost no hemolysis reaction was observed. However, as the concentration increased, the hemolysis rate gradually increased (Fig.  3 D). In contrast, XYP1-18 caused almost no hemolysis at concentrations ranging from 1.25 μM to 80 μM, and only minor hemolysis occurred even at the highest concentration (160 μM). XYP1-18-1 had low hemolysis at 1.25 μM to 160 μM, with a maximum hemolysis rate of only 20% (Fig.  3 D). It was indicated that XYP1-18 and XYP1-18-1 were safer than XYP1 over a wider range of concentrations.

The efficacy of XYP1-18 in vivo was comparable to XYP1

The survival experiment was used to evaluate the therapeutic efficacy of several peptides against T. gondii infection in vivo. All mice in the control group died at 127.1 h. In contrast, XYP1 can significantly prolong the survival of mice to 208.5 h, XYP1-18 can be extended to 191 h, and XYP1-18-1 can only be extended to 145 h (Fig.  3 E). There is no significant difference between XYP1-18 and XYP1. XYP1-18-1 group had poor effect on prolonging the survival time of mice.

XYP1-18 and XYP1-18-1 can destroy the surface structure and organelles of T. gondii

The effects of XYP1, XYP1-18, and XYP1-18-1 on the surface structure of T. gondii tachyzoites were observed using SEM. Normal T. gondii tachyzoites were crescent-shaped, with a sharp front end and a rounded back end (Fig.  4 A, B ). The cell membrane was complete and there were many micropores on the surface. After being treated with 10 μM polypeptides for 2 h, various damages were observed on the surface membranes of tachyzoites. In the XYP1 group, pores formed on the cell membrane surface, and many tachyzoites occurred wrinkling or expansion of the membrane (Fig.  4 C, D ). Under the influence of XYP1-18, many tachyzoites disintegrated, with depressions appearing on the cell membrane surface (Fig.  4 E, F ). XYP1-18-1 caused the pores or depressions on the surface of the T. gondii cell membranes, and some tachyzoites disintegrated (Fig.  4 G, H ). These results indicated that the three peptides had varying degrees of impact on the surface morphology of T. gondii . Specifically, XYP1-18 and XYP1-18-1 had more pronounced destructive effects on T. gondii than XYP1.

The SEM results of XYP1, XYP1-18, and XYP1-18-1 effects on the surface structure of T. gondii tachyzoites. A , B T. gondii tachyzoites were incubated with PBS buffer for 2 h. C , D T. gondii tachyzoites were incubated with XYP1 (10 μM) solution for 2 h. E , F T. gondii tachyzoites were incubated with XYP1-18 (10 μM) solution for 2 h. G , H T. gondii tachyzoites were incubated with XYP1-18-1 (10 μM) solution for 2 h. ↓: micropore; → : holes; ☆ : shrunken or sunken tachyzoites of T. gondii ; *: disintegrating T. gondii tachyzoites; enlarged tachyzoite. Scale: A  = 10 μm; B  = 2 μm; C  = 10 μm; D  = 2 μm; E  = 5 μm; F  = 2 μm; G  = 5 μm; H  = 2 μm

TEM was used to observe the organelle structure inside T. gondii . The organelles inside the untreated tachyzoites were intact (Fig.  5 A, B ). In the XYP1 group, the tachyzoites not only showed ruptures in the surface membrane, but also different degrees of vacuolization internally (Fig.  5 C, D ). With the treatment of XYP1-18, the majority of tachyzoites exhibited not only severe deformation and rupture of the surface membrane, but also serious shrinkage and vacuolization internally (Fig.  5 E, F ). In the XYP1-18-1 group, the internal structure of tachyzoites was even more severely damaged, with not only shrinkage and vacuolization, but also complete dissolution of the tachyzoites observed (Fig.  5 G, H ). In conclusion, these polypeptides had varying degrees of impact on the internal structure of T. gondii tachyzoites.

The TEM results of XYP1, XYP1-18, and XYP1-18-1 effects on the internal structure of tachyzoites of T. gondii. A , B T. gondii tachyzoites were incubated with PBS buffer for 2 h. C , D T. gondii tachyzoites were incubated with XYP1 (10 μM) solution for 2 h. E , F T. gondii tachyzoites were incubated with XYP1-18 (10 μM) solution for 2 h. G , H T. gondii tachyzoites were incubated with XYP1-18-1 (10 μM) solution for 2 h. Co: conoid; Dg: electron-dense granule; Go: Golgi complex; Lb: lipid body; Nu: nucleus; Nm: nuclear membrane. Scale: A  = 2 μm; B  = 2 μm; C  = 2 μm; D  = 2 μm; E  = 10 μm; F  = 2 μm; G  = 10 μm; H  = 5 μm

XYP1 affected the inflammatory and immune responses of host cells

Previous studies have initially explored the effects of XYP1 on T. gondii -related genes and pathways. To better understand the host cell response to T. gondii under the influence of XYP1, changes in genes and pathways before and after XYP1 treatment were analyzed by transcriptome technology. P  < 0.05 and fold change > 2 were used to determine whether genes in the control group and the XYP1 group showed differential expression. A total of 65 genes had significant differences in expression, including 7 up-regulated genes and 58 down-regulated genes (Fig.  6 A). The M-versus-A (MA) map and volcano map represented significantly differential genes under differential screening conditions (Fig.  6 B, Additional file 1 : Fig. S3A). After clustering analysis of the differentially expressed genes (Fig.  6 C), it was observed that the samples from the control group and the XYP1 group clustered together in the same cluster, indicating that these genes exhibited similar expression patterns in the two sample groups and may have similar biological functions.

Effects of XYP1 on genes and pathways associated with host cells infected with T. gondii. A The number of differential genes up-regulated and down-regulated. B In the differentially expressed volcano map, green and red dots represent significantly different genes, and gray dots represent nonsignificantly different genes. C The results of cluster analysis for each difference group. Red indicates highly expressed genes, blue indicates genes that are low expression. D GO enrichment analysis Top30 bar chart. The horizontal coordinate is the GO entry name and the vertical coordinate is −log 10 P -value. E Distribution of differentially expressed genes and all genes in KEGG pathway. The horizontal axis is the ratio (%) between the genes annotated to each metabolic pathway and the total number of genes annotated to the KEGG pathway. F Bubble diagram of KEGG enrichment Top20, entries with larger bubbles contain more differential protein coding genes, and the redder the bubble color, the greater the difference

The 37 differentially expressed genes were classified into 3 main gene ontology (GO) categories, which were further assigned to 64 specific functional subcategories. As shown in Additional file 1 : Fig. S3B, a total of 448 significantly enriched GO terms were identified. Among them, 310 terms were enriched in biological process (BP), 65 terms in cellular component (CC), and 73 terms in molecular function (MF). In the top 30 GO terms (Fig.  6 D), the most significantly enriched term in BP was “inflammatory response,” in CC it was “vesicle,” and in MF it was “GTPase activity.” The KEGG enrichment results were summarized in Fig.  6 E. Immune-related pathways with a higher gene enrichment included the IL-17 signaling pathway, tumor necrosis factor signaling pathway, cytokine-cytokine receptor interaction, NOD(Nucleotide oligomerization domain)-like receptor signaling pathway, and NF-kappa B signaling pathway. The top 20 significantly enriched pathways were selected from the KEGG enrichment results for display in a bubble chart, as shown in Fig.  6 F.

Furthermore, protein–protein interaction (PPI) networks were mapped and the core interaction networks were predicted. As shown in Fig.  7 A, a darker green color indicates higher score of the nodes. MCC, DMNC, and radiality algorithms are used to screen the Top20 hub proteins (Fig.  7 B–D). The redder the color is, the higher the score of the nodes, and the more yellow–green the color is, the lower the score. In the MCC algorithm, the top three proteins were IL1B, IL-6, and CXCL-8; in the DMNC algorithm, the top three proteins were CCL-8, CSF3, and CXCL3; in the radiality algorithm, the top three proteins were IL1B, IL-6, and CXCL-8, consistent with the MCC results. Subsequently, the intersection was taken and a Venn diagram was plotted (Fig.  7 E), showing 19 common hub proteins (Additional file 1 : Table S2). MMC and radiality shared IL1B. There is ENTPD1 in the DMNC algorithm that has no intersection with the other two algorithms. The results of qPCR showed that the transcription levels of IL-17 and TP53 were down-regulated and NF-kB was up-regulated in the XYP1 treatment group (Fig.  7 F–H).

Differential expression protein interaction network analysis and qPCR validation. A Circular protein interaction network map of differentially expressed genes, the darker the green color, the higher the node score. B The top 20 hub genes based on MMC algorithm. C The top 20 hub genes based on DMNC algorithm. D The top 20 hub genes based on radiality algorithm. E Venn diagram of hub genes intersection of three algorithms. F – H Changes in transcription levels of IL-17 , tp53 , and NF-κB genes

When T. gondii infects humans, it often remains inapparent, but those with congenital infections or compromised immune systems may suffer severe symptoms [ 34 , 35 ]. The development of novel drugs is a current research focus, and venom peptides derived from animal toxins hold great potential as scaffolds for clinical applications, including cysteine-stabilized peptides and linear helical peptides [ 36 ]. In this study, eight derived peptides were successfully generated by amino acid truncation and replacement of XYP1, which originates from the venom of Lycosa coelestis . Preliminary results suggested that XYP1 might exert anti- T. gondii effect on the basis of an α-helical structure [ 13 ]. Phospholipids serve as a major source of negative charge in cell membranes, and active peptides with an α-helical structure often target phospholipids [ 37 ]. Therefore, the α-helix structure of the derived peptide is preserved. Because these peptides contain positively charged amino acid side chains, they can interact with phospholipids on the cell membrane by electrostatic interaction. The amphipathic α-helical structure formed by the peptides facilitates their insertion into the cell membrane [ 38 ]. When the peptide inserts into the cell membrane, its hydrophobic regions interact with the phospholipid bilayer, disrupting the integrity of the cell membrane, which leads to the formation of pores and gaps. These pores and gaps facilitate the leakage of cellular contents, thereby disrupting cellular homeostasis [ 39 , 40 ].

Another essential property of AMPs is amphipathy, which refers to the relative abundance of hydrophilic and hydrophobic residues. Studies have shown that indolicidin analogs with increased amphipathy and charge displayed lower hemolytic activity and maintained antimicrobial activity [ 41 , 42 ]. The distribution of hydrophobic and hydrophilic residues on both sides of the polypeptide plays a crucial role in the biological activity and bioavailability of AMPs [ 17 ]. By replacing individual amino acid residues, we successfully generated four new peptides that were structurally similar to truncated peptides but had more evenly distributed hydrophilic and hydrophobic residues, which may enhance their biological activity.

The initial screening experiments revealed that three peptides, XYP1-17, XYP1-18, and XYP1-18-1, exhibited much strong anti- T. gondii effects than the control group. Among them, XYP1-18 had similar anti- T. gondii activity to XYP1, while the other peptides had minimal effects. The lack of efficiency in XYP1-15 and XYP1-16 could be attributed to the absence of key functional residues induced by a greater number of truncated amino acids. Cationic amino acids, such as lysine (Lys) or arginine (Arg), are crucial for antimicrobial activity in peptide sequences. These common cationic amino acids are typically positioned on the hydrophilic side of the antimicrobial peptides [ 43 , 44 , 45 ]. There is a complex interaction among net charge, hydrophobicity, and amphipathicity [ 46 , 47 , 48 , 49 ]. These parameters play different roles in various peptide sequences, with optimal antimicrobial activity requiring the right balance of hydrophobicity, amphipathicity, and charge density. The truncated XYP1-15 and XYP1-16 significantly increased hydrophobicity and decreased net charge, while excessive hydrophobicity led to increased cytotoxicity and loss of activity, and decreased net charge may lead to decreased activity. Although the amphiphilicity and net charge of XYP1-15-1 and XYP1-16-1 increased, their hydrophobicity decreased, which may affect their anti- T. gondii activity.

The main difference between XYP1-17-1 and XYP1-18-1 were the lack of a leucine residue. Typically, higher hydrophobic leucine (Leu, L) can be substituted for lower hydrophobic amino acid residues to compensate for this loss [ 50 , 51 , 52 ]. Leucine side chains are relatively large, containing an isopropyl group that contributes to hydrophobic effects. These effects are crucial in protein folding and maintaining the structure of α-helices [ 53 ]. Further in vitro screening experiments had shown that XYP1-18 and XYP1-18-1 were the most effective derived-peptides, with lower cytotoxicity and hemolytic activity than XYP1, thus they have better potential as candidates for polypeptide therapy in toxoplasmosis.

We previously discovered that XYP1 can significantly down-regulate the MIC10 , HSP29 , and rpb-10 genes in T. gondii , which was associated with membrane component, invasion, and proliferation of T. gondii . However, it remains unclear how XYP1 affects the interactions between T. gondii and the host. In this study, RNA-seq was used to focus on the differential genes in the host before and after XYP1 treatment. In total, 65 genes showed significant differences in expression, with 7 up-regulated (10.77%) and 58 down-regulated (89.23%) genes. According to GO analysis, the most enriched biological process is the inflammatory response. Genes involved in this process include ADORA2A , NFKBIZ , ELF3 , and TNIP3 . This suggested that XYP1 may eliminate T. gondii by modulating the host cell’s immune system to trigger inflammatory response and immune response. When host cells were infected with T. gondii , it activated the immune system, leading to the release of inflammatory cytokines such as IL-6, IL-1β, IL-12, and TNF-α. These cytokines promote inflammation, activating the host cell’s immune function to combat T. gondii [ 54 , 55 , 56 , 57 , 58 ]. The NF-κB is a driver of inflammation response, and NFKBIZ is an inhibitor of NF-κB [ 59 ]. Our findings revealed that XYP1 may kill T. gondii by up-regulating the NF-κB regulated pathway. Additionally, on the basis of KEGG analysis, we found that the most significantly enriched pathways among down-regulated genes were the IL-17 signaling pathway, cytokine–cytokine receptor interaction, and TNF signaling pathway. The down-regulation of IL-17 was verified by qRT-PCR assay (Fig.  7 F). IL-17 production was associated with protective and pathogenic responses during T. gondii infection [ 60 ]. Thus, XYP1 might down-regulate the host cell IL-17 signaling pathway, reducing host pathological responses. These pathways were crucial for regulating various biological processes via cell-to-cell interactions and signals.

To sum up, the study of XYP1-derived peptides provided a promising treatment approach for T. gondii infection, with great potential for clinical application. While considerable progress has been made, there remain hurdles to overcome. For example, the use of peptide drugs has limitations such as a short half-life. Therefore, future research should focus on improving the properties and effectiveness of peptides to enhance their clinical value. Additionally, further investigation is needed to explore how XYP1 specifically regulates the immune response in host cells to better understand its anti- T. gondii mechanisms. In summary, this study lays the groundwork for the development of new anti- T. gondii drugs and offers important insights for the research and usage of peptide-based drugs.

This study successfully identified two derived peptides, XYP1-18 and XYP1-18-1, with anti- T. gondii activity through modifying XYP1. In comparison with the parent XYP1, these peptides exhibited lower cost and improved safety (low cytotoxicity and hemolytic activity). Survival assay results demonstrated that XYP1-18-1 also can prolong the survival time of T. gondii -infected mice. Furthermore, XYP1 can eliminate T. gondii by modulating the inflammatory and immune response of host cells. These discoveries provided a novel strategy and direction for treating T. gondii infection, as well as useful references for the development and application of peptide-based therapies.

Availability of data and materials

The HPLC and ESI–MS results of peptides generated during the current study is stored in the Figshare repository, ***:. The RNA-Seq data have been deposited in the Genome Sequence Archive (Genomics, Proteomics & Bioinformatics 2021) in National Genomics Data Center (Nucleic Acids Res 2022), China National Center for Bioinformation/Beijing Institute of Genomics, and Chinese Academy of Sciences (GSA-Human: HRA007277), and are publicly accessible at https://ngdc.cncb.ac.cn/gsa-human .

Abbreviations

Antimicrobial peptide

C-C motif chemokine

Colony-stimulating factor

C-X-C motif chemokine ligand

Dulbecco’s modified Eagle’s medium

Dimethyl sulfoxide

Electrospray Ionization mass spectrometry

Fetal bovine serum

Human foreskin fibroblast cell

High-performance liquid chromatography

Interleukin

Optical density

Phosphate buffered saline

Scanning electron microscope

Transmission electron microscope

Tumor protein 53

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Acknowledgements

We express our gratitude to all participants involved in this research.

The work was supported by the National Natural Science Foundation of China (32170510) and Hunan graduate research innovation project (QL20230021).

Author information

Jing Li and Kaijuan Wu contributed equally to this work.

Authors and Affiliations

Department of Parasitology, School of Basic Medical Sciences, Central South University, Changsha, 410013, Hunan, China

Jing Li, Kaijuan Wu, Xiaohua Liu, Dongqian Yang, Jing Xie, Yixiao Wang, Kang Liu & Liping Jiang

China-Africa Research Center of Infectious Diseases, Central South University, Changsha, 410013, Hunan, China

Liping Jiang

Department of Vascular Surgery, The Third Xiangya Hospital, Central South University, Changsha, 410013, Hunan, China

Hunan Key Laboratory of Traditional Chinese Veterinary Medicine, Hunan Agricultural University, Changsha, 410128, China

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Contributions

L.J., J.L., and K.W. participated in the conception and design of this study. J.L., K.W., X.L., D.Y., J.X., Y.W., and K.L. performed the experiments and analyzed the results. Z.W. and W.L. provided guidance and advice on transcriptome analysis. J.L. and K.W. wrote the manuscript. L.J. reviewed the manuscript and acquired the funding support. All authors made contributions to the final version and approved the submission.

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Correspondence to Liping Jiang .

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This study was approved by the Ethics Committee of the School of Basic Medical Science, Central South University, Changsha, China (protocol code: 2020KT-11), and complied with national regulations on the ethical treatment of experimental animals.

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Additional file 1: table s1..

The properties and parameters of XYP1 and its derived peptide. Table S2. The hub proteins of intersection of three algorithms. Figure S1. The ESI–MS results of XYP1-15, XYP1-15-1, XYP1-16, XYP1-16-1. The abscissa is the mass charge ratio (m/z), and the ordinate is the ionic strength. Figure S2. The ESI–MS results of XYP1-17, XYP1-17-1, XYP1-18, XYP1-18-1. Figure S3. GO enrichment analysis of differential genes. (A) MA map of differentially expressed genes. The X -axis is the normalized average expression of genes in all samples involved in the comparison, and the Y -axis is log2Fold Change. Red indicates significant differential genes. (B) Comparison map of differentially expressed genes and distribution of all genes at GO level. Blue represents all the gene-enriched GO entries, red represents the differential gene-enriched GO entries, horizontal axis represents the entry name, and vertical axis represents the number of genes corresponding to the entry and its percentage.

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Li, J., Wu, K., Liu, X. et al. Anti- Toxoplasma gondii effects of XYP1-derived peptides and regulatory mechanisms of XYP1. Parasites Vectors 17 , 376 (2024). https://doi.org/10.1186/s13071-024-06455-7

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Received : 28 May 2024

Accepted : 16 August 2024

Published : 04 September 2024

DOI : https://doi.org/10.1186/s13071-024-06455-7

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• XYP1-derived peptides
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• Transcriptome analysis

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