Class 12 Physics Interference Notes

Class 12 Physics Interference Notes



Unit – 3
Wave and optics
Chapter – 11
Interference 

 


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Coherent source

Two sources are said to be coherent if they emit a light wave of the same wavelength, same amplitude and they are in the same phase with each other having a constant phase difference

It is not possible two independent sources are coherent but experimentally two virtual sources come from a single source that acts as a coherent source

Interference

When two waves of the same frequency having constant, phase difference traveling in the same direction will meet, they produce a resultant wave by superposition principle. Thus the modification in the intensity of light due to the superposition of two or more waves is called interference.

Types of interference

1. Constructive interference

If two waves having the same frequency and same phase overlap with each other, the amplitude of the resultant wave means a resultant wave of more intensity will form. Such interference is called constructive interference.

Constructive Interference

Fig: Constructive Interference

∴path difference = nλ

Destructive Interference

If two waves of the same frequency and same amplitude travel in the same direction with opposite phases to each other then a resultant wave of zero amplitude is formed. Such interference is called destructive interference in which the intensity of light will be minimum.

Destructive Interference

Fig: Destructive Interference

∴path difference = (n + 1/2) λ

Condition to produce interference

1) Two-Source of light must be coherent

2) Light source used must be monochromatic

3) Two sources of light must be closed to each other

4) The distance between the coherent source of the light and the screen should be large.

5) Coherent source of light must be narrow

6) Coherent source of light must emit waves continuously.

Young’s double-slit experiment

Young double slit experiment

Fig:-Young double-slit experiment

Let ‘s’ be the monochromatic source of light having wavelength ‘λ’. Also, let S1 and S2 be the two-slit which are equal distance from source ‘s’ act as coherent sources suppose ‘D’ be the distance between source as know at which path

Also, suppose ‘C’ be the center of the screen at which the path difference is zero. Hence at point ‘C’ maximum intensity is observed. Also, let ‘P’ be any point at distance ‘I’ from ‘C’.

Class 12 Physics Interference

Also,

Class 12 Physics Interference

If ‘P’ is very close to C

S2P ≈ S1P ≈ D

Class 12 Physics Interference

Bright Frings

The point ‘p’ has a maximum,

Bright Frings

Which gives the distance of n^th bright fringe from center

Bright Frings

Which is the width of bright fringes

Dark fringes

The point ‘P’ has maximum

Dark fringes

Which gives the distance of n^th dark fringe from center

For n = 0

Dark fringes

For n = 1,

Dark fringes

Dark fringes


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