Interference of Light for Engineering Physics B.Tech 1st Year

Interference of Light:

Definition:  When two light waves from different coherent sources meet together, then the distribution of energy due    to one wave is disturbed by the other. This modification in the distribution of light energy due to super-    position of two light waves is called “Interference of light.

Conditions for Interference:

1. The two sources of light should emit continuous waves of same wavelength and same time period i.e. the source should have phase coherence.
2. The two sources of light should be very close to each other.
3. The waves emitted by two sources should either have zero phase difference or no phase difference.

Coherent Sources:

Those sources of light which emit light waves continuously of same wavelength, and time period,    frequency and amplitude and have zero phase difference or constant phase difference are coherent sources.

Types of Interference:

There are two types of interference.
Constructive interference.
Destructive interference.

Constructive interference:

When two light waves superpose with each other in such away that the crest of one wave falls on the crest of the second wave, and trough of one wave falls on the trough of the second wave, then the resultant wave has larger amplitude and it is called constructive interference.

Conditions for Constructive Interference:

For constructive interference, path difference between two waves is ml
   i.e. path difference = ml
or path difference = 0, l, 2l, 3l, 4l, …..
where m = order = 0, +_1, +_2, +_3,…..

Effects of Constructive interference:

In constructive interference, two waves of light reinforce each other.
In constructive interference, a bright fringe is obtained on the screen.

Destructive interference:

When two light waves superpose with each other in such away that the crest of one wave coincides the trough of the second wave, then the amplitude of resultant wave becomes zero and it is called destructive interference.

Conditions for Destructive Interference:

If the path difference between two light waves is (m+1/2)l , then the interference between them will  be destructive.
 Path difference = (m+1/2)l
i.e. path difference = 1/2l, 3/2l, 5/2l , ….
where m = order = 0, +_1, +_2, +_3,…..

Effects of Destructive Interference:

In destructive interference, two waves cancel the effects of each other.
Due to destructive interference a dark fringe is obtained on the screen.

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