## Question 6: What is the principle of interference of light?Discuss the necessary conditions for the interference of light.

ANSWER

Suppose two waves of light having the same frequency, amplitude and a constant phase relationship (may be equality or difference) are traveling through a certain region of space at a certain time in the same direction. Then they will reinforce each other at certain points and will cancel one another effect at certain other points. This phenomenon of superposition of light waves is called interference of light. Therefore;

*The effect produced by the superposition of two/more waves from coherent sources passing through the same region of space is called interference of light. *

In order to observe the phenomena, we obtain monochromatic light waves from two coherent sources and let them interfere on a screen behind the sources. Bright fringes (bands) are observed where the two waves reinforce each other and black bands are observed where the two waves cancel one another.

### Types of interference

There are two types of interference.

#### Constructive Interference

If the waves reach a certain point in phase, we see a constructive interference there. The waves reinforce one another and brightness is seen at the point. At such points the path difference is 0, 2π, 4π, 6π…. As phase change 2π = λ, therefore, the interference is constructive for a path difference of 0, λ, 2λ, 3λ…. So if ‘d’ is the path difference, then the condition for constructive interference is;

d = 0, λ, 2λ, 3λ …

or d = mλ

Where m = 0, 1, 2, 3…

#### Destructive interference

Points at which the two waves meet in opposite phase, they cancel the effect of one another and destructive interference takes place. On the screen we see dark fringes at such points.

The phase difference for such waves is π, 3π, 5π, ………., corresponding to λ/2 , 3λ/2 , 5λ/2 , ……..

### Conditions for the interference of light

Following are the condition of interference of light.

- Light waves must come from the coherent sources.
- The amplitude of the waves must be equal or nearly equal.
- Light waves should be perfectly monochromatic.
- Path difference for the superposing waves must be small.
- Principle of linear superposition must be applicable.

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