# What is the Young Double slit experiments? What is the conditions for minima and maxima?

## Young Double slit experiments:

The superposition of two light waves having zero phase difference or no phase difference is known as Interference of light. There are two types of interference possible i.e. constructive interference (superpose in same phase) and destructive interference (superpose in opposite phase). For the explanation of interference phenomenon the Thomas Young in 1803 perform an experiment which is nowadays referred to as Young double slit experiment.

In Young’s experiment, one source is split into two very narrow parallel slits, separated by a distance d, are cut into a thin sheet of metal. So both the slit pass monochromatic light; this monochromatic light from a distant light-source, passes through the slits and eventually hits a screen a comparatively large distance L from the slits; and form an inference pattern on screen. If the waves are completely in phase then constructive interference occurs, resulting in a light patch on the screen; on the other hand if the waves are 1800 out of phase then destructive interference occurs, resulting in a dark patch on the screen. The experimental setup is sketched in Figure.

## Phase difference and Path difference

When the two rays travel in constant phase difference and the difference between optical path of both wave is known as path difference.

If we consider two waves which is originated from two source S1 and S2 meet at point D on the screen and form an interference pattern; the path difference between two wave at a point P is given as:

• Path difference = S2D-S1D
• Phase difference d = 2π/λ X (path difference)

## Calculation of Intensity of two interfere fringes

Let us consider a light source emit a monochromatic light which split into two waves with λ wavelength by S1 and S2 source. Both waves have A1 and A2 amplitudes meet at point D with S1D and S2D path. If the path difference is d then the displacement of both waves is given by

• ### For Constructive interference or condition for maxima

For the value of cosd lies between ±1

For maxima condition cosd should be = +1

From equation (6)

I = 2A2 (1+1)

Imax= 4A2

This is maximum intensity expression for constructive inference

• ### For destructive interference or condition for minima

For minima condition cos d should be = -1

From equation (6)

I = 2A2 (1-1)

IMin = 0

This is minimum intensity expression for destructive inference.

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