air wedge experiment diagram

Engineering Physics: Unit III: b. Optics

Theory of air wedge, definition, experiment, description, applications.

Air-wedge arrangement is used to find the thickness of a thin sheet or a wire. It is also used to test the planeness of the glass plate.

THEORY OF AIR WEDGE AND EXPERIMENT

A wedge shaped (V-shaped) air film enclosed in between two glass plates is called air wedge.

Theory of air wedge experiment : 

When two optically plane glass plates ( A & B ) are inclined at a very small angle 0, a wedge shaped thin air film is formed between the surfaces as shown in fig. 4.11. The thickness of the air film increases outwards from the line of contact 'O' of the glass plates.

The light rays from a monochromatic light source is made to fall perpendicularly on the film.

The incident rays of light is partially reflected from the upper surface of the air film and partially reflected from the lower surface of the air film.

These two reflected rays will interfere and a large number of straight alternative bright and dark fringes are formed.

If t is the thickness of the air film corresponding to the n th dark band with wedge angle o at a distance of x metre from the edge of contact, then the path difference between the two reflected rays (Fig 4.12)

For air film, refractive index of the film μ = 1

cos r =1, since angle of incidence is very small, so angle of refraction is also very small ie., r = 0; cos θ = 1 

Now, 2 t = n λ        ….(2)

where λ - wavelength light

Since x is the distance of the n th dark band from the edge of contact O,

substituting equation (3) in equation (2), for the n th dark band

Similarly, for the next dark band ie., (n + 1) th dark band 

2( x + β)θ=( n + 1)λ          ... (5)

where β is the fringe width

subtracting equation (4) from equation (5), we have

The same relation is obtained if we consider the bright fringe.

Thickness of a thin wire and very thin foil

The given wire whose thickness d is to be measured is placed inbetween the two glass plates to form a wedge shaped air film.

Now if l is its distance from the edge of contact (length of the wedge), then from fig 4.13.

Substituting eqn (7) in (6)

Thus, thickness of very thin specimen can be determined by using the interferance technique in wedge shaped film.

Applications of air-wedge 

Determination of diameter (thickness) of a wire or thickness of a thin sheet of paper (Experiment)

An airwedge is formed by keeping two optically plane glass plates in contact along one of the edges and a thin wire near the other end, parallel to the contact edges of the glass plates.

Therefore, glass plater are inclined at a very small angle (one end of these two glass plates may be tied using a thread or a rubber band). This is called airwedge arrangement

Description

This arrangement is kept on the bed of the travelling microscope (Fig 4.14).

A parallel beam of monochromatic light from a light source is reflected down on the air wedge by a glass plate kept inclined at an angle 45° to the horizontal.

Interference takes place between the light reflected at the top and bottom surfaces of the air film between the two glass plates.

Interference pattern (Fig 4.15) consisting of a series of bright and dark bands of equal width is viewed by a travelling microscope arranged above the airwedge.

Microscope is focussed on these fringes and the vertical cross wire is made to coincide with n th bright band near the edge of contact of the glass plates.

The reading on the horizontal scale of the microscope is noted. The cross wire is made to coincide with successive 5 th fringes ( n +5, n + 10 ... n + 40) and the corresponding microscope readings are noted. The readings are recorded in the table 4.1.

From the table, the average fringe width β is determined. Using the microscope, the distance l between the edge of the contact and the wire is also measured.

Knowing the wavelength of the monochromatic light source, the thickness of the wire is found out using the formula.

Engineering Physics: Unit III: b. Optics : Tag: : Definition, Experiment, Description, Applications - Theory of Air Wedge

Related Topics

Optics - Introduction

Reflection of light waves - Laws of reflection

Refraction of light waves - Laws of refraction, Refractive index

Total Internal Reflection - Definition, Equation, Applications

Interference of light waves

Theory of Air Wedge - Definition, Experiment, Description, Applications

Michelson's Interferometer - Operational Principle, Construction, Diagram, Working, Formation of fringes, Types of Fringes, Applications

Solved Problems of Optics - Engineering Physics

Two Marks Questions with Answers - Optics | Engineering Physics

16 Marks Questions - Optics | Engineering Physics

Assignment Problems - Optics | Engineering Physics

Related Subjects

Engineering Physics

PH3151 1st semester | 2021 Regulation | 1st Semester Common to all Dept 2021 Regulation

Interference of Light Waves

Light Waves as Measuring Units

Here we have two stacked glass squares. One is attached to the blue solid metal shaft, and the other to the channel iron. There is a Mercury light source that shines on the glass blocks when it is turned on.

WHAT TO DO:

• Push the small red button on the grey box to turn the light on .
• Look at the small glass square below the large red knob. You should see a green light with dark bands.
• Push down on the large red knob.

WHAT HAPPENS TO THE SPACING OF THE DARK AND BRIGHT BANDS IN THE SQUARE?

The spacing of the bright and dark bands changes!

What is going on?

The two glass squares are separated by ~0.001 in leaving a thin film air wedge between them. Pushing on the red knob bends the channel iron slightly moving the top glass square down, while the blue shaft is rigid and doesn’t allow the second glass square to move.

The alternating dark and bright lines, called fringes , result from the interference pattern of light reflected from the upper and lower sides of the air wedge.  Moving the plates closer together or farther slightly changes the thickness of the air wedge, changing the interference pattern, and the fringes appear to move sideways. The displacement of one fringe is equivalent to a shift of ½ of the wavelength of the Mercury light source (λ Mercury =0.00027mm).

Wedge fringes

If two glass plates are placed face to face with one end separated by a piece of tissue paper or thin metal foil an air wedge will be formed between them. If monochromatic light is shone on the plates a series of straight-line fringes will be seen parallel to the line along which they touch (Figure 1). This is due to interference by division of amplitude, as with Newton's rings. Some light is reflected from the bottom surface of the top plate and some from the top surface of the bottom plate. To see the fringes clearly the angle must be small, something like 4 minutes of arc. You should also look for fringes close to the join of the plates where the air gap is smallest, since the fringes are not well defined for path differences of more than some hundred wavelengths (0.058 mm for sodium light - compare this with the thickness of a sheet of paper). Consider a point a distance x from the join. Path difference = 2e = 2xθ where θ is the angle between the plates in radians (this angle is small, so tan θ = θ in radians). For an air wedge there is a phase change on reflection at the top surface of the lower plate and so:

The travelling microscope or the eye must be focused close to the upper surface of the air wedge since this is where the fringes are localised. Pressing down gently with your finger on the plates will move the interference pattern, since only a very small movement is needed to alter the path difference significantly. The vertical soap film is a good example of wedge fringes. As the soap drains to the bottom of the film a wedge of very small angle is formed. When the top part goes black the film is about to break. The flatness of a glass surface may be tested by placing it on a test surface which is known to be flat and illuminating them with monochromatic light; any imperfections will show up as loop-shaped interference fringes around bumps or depressions on the surfaces.

Possible air wedge configurations: ͑ a ͒ Two flat-parallel glass plates. ͑ b ͒ Two glass wedges. ͑ c ͒ Two glass prisms. ͑ d ͒ Two-prism beam splitter. 

Contexts in source publication

Similar publications.

• S. A. Ambrozevich

• J APPL PHYS

• Affiliations
• Michael A. Saville
• J PHYS D APPL PHYS

• PLASMA PHYS REP+

• A. E. Ter-Oganesyan
• A. R. Mingaleev
• S. A. Pikuz

• S. M. Zakharov

• OPT EXPRESS

• Mikhail Medvedev
• Alexandr Selyukov
• PHYS PLASMAS

• W. M. Potter

• G V Ivanenkov

• A S Selyukov
• Recruit researchers
• Join for free
• Login Email Tip: Most researchers use their institutional email address as their ResearchGate login Password Forgot password? Keep me logged in Log in or Continue with Google Welcome back! Please log in. Email · Hint Tip: Most researchers use their institutional email address as their ResearchGate login Password Forgot password? Keep me logged in Log in or Continue with Google No account? Sign up