Optics Interference: Important part-A 2 marks Short Questions and Answers

ANNA UNIVERSITY PART‒A QUESTIONS & ANSWERS

1. Explain why colours are seen over a thin film of petrol on roads.

When white light, (sunlight) falls on a thin film of petrol on roads, interference takes place between the reflected light from the top and bottom layer of the petrol film and thus it appear coloured.

NOTE: The same principle is applicable for the formation of colours in soap bubbles.

2. How is optical flatness of a surface checked using interference phenomena?

(Or)

What is the principle of using air wedge method to test the planeness of a glass plate?

The material (OB) whose flatness has to be tested is kept over a glass plate (OA) and tied at one end (O). Thus an airwedge is formed between the two glass plates. Now, light is made to fall over the setup and the interference pattern is viewed through a microscope.

i) If the bands appear regularly spaced, the surface is flat (Fig. 8a)

ii) If the bands are curved towards the contact edge, the surface is concave (Fig. 8b)

iii) If the bands are curved away from the contact edge, the surface is convex (Fig. 8c)

ADDITIONAL PART‒A QUESTIONS AND ANSWERS

1. What is meant by interference of light?

When two light waves from the sources moves in the same direction, then the light wave trains from these superimpose on each other, which results in the modification of distribution of intensity. This modification of light energy due to the superposition to two waves is known as interference.

It the amplitudes of the two waves increase the intensity of the resultant, then it is called as constructive interference. If there is decrease in intensity of the resultant, then it is called as destructive interference.

2. State Superposition Principle.

It states that, the resultant displacement of the particle of a medium acted upon by two or more waves simultaneously is equal to the algebraic sum of the displacement of the particle due to the individual waves.

3. What are conditions to be satisfied for interference to occur.

1. There must be atleast two coherent sources.

2. The distance between the sources must be very less.

4. The distance between the source and the screen should be made as large as possible.

5. For constructive interference, the path difference should be equal to nλ and for destructive interference, it should be (2n+1)λ/2 .

4. What is meant by fringe width?

The distance between any two consecutive bright or dark fringes is known as fringe width β. It is constant for both bright and dark fringe. It is given by

 β = λD / d

5. What is meant by an Air wedge? Give its usage.

When two plane glass plates of equal thickness and diameter is tied at one end, then the two plane surfaces is inclined at an angle θ. Thus a wedge shaped air film is formed between the two glass places and hence called as an Air wedge. It is used to find the thickness of very thin materials like our hair, paper etc.,

6. What is the use of compensating plate in the Michelson interferometer?

In Michelson interferometer, the ray from the source and the ray which suffers reflection from mirror M₁ is passed through the glass plate G₁ twice whereas the ray reflected from mirror M₂ does not even passes once through the glass plate G₁. So to compensate the path a second glass plate G₂ is introduced between the glass plate G₁ and the mirror M₂ as shown in Fig. 8(d).

7. What are the uses of Michelson interferometer ?

It is used to determine the

(i) wavelength of a given monochromatic source

(ii) the difference between the two neighbouring wavelengths or resolution of the spectral lines

(iii) refractive index and thickness of various thin transparent materials and

(iv) for the measurement of the standard metre in terms of the wavelength of light.