Describe Young's double slit experiment to produce interference pattern due to a monochromatic source of light. Deduce the expression for the fringe width.
Conditions for Young’s double slit experiment are:
(i) The sources should be monochromatic and originating from common single source.
(ii) The amplitudes of the waves should be equal.
Let S1 and S2 be two coherent sources separated by a distance d.
Let the distance of the screen from the coherent sources be D. Let point M be the foot of the perpendicular drawn from O, which is the midpoint of S1 and S2 on the screen. Obviously point M is equidistant from S1 and S2. Therefore the path difference between the two waves at point M is zero. Thus the point M has the maximum intensity.
Now, Consider a point P on the screen at a distance y from M. Draw S1N perpendicular from S1 on S2 P. 
The path difference between two waves reaching at P from S1 and S2 is given by, 
Since, D >> d, 
Expression for Fringe Width:
Distance between any two consecutive bright fringes or any two consecutive dark fringes are called the fringe width. It is denoted by 
.
Let, 
be the distance of two consecutive fringes. Then, we have
So, fringe width is 
Fringe width is same for both bright and dark fringe.
