Question

In Young's experiment, the monochromatic light is used to illuminate two slits S₁ and S₂ as shown. Interference fringes are observed on a screen placed in front of the slits. Now if a thin glass plate is placed normally in the path of beam coming from the slit S₁, then

(a) fringe width will decrease 

(b) fringes will disappear 

(c) fringe width will increase 

(d) there will he no change in fringe width.

Answer 1

The correct option is (d).

Explanation:

Suppose S₁ and S₂ are the slits at a distance d from each other. Distance of screen be D. Let P be a point where there is a bright fringe. A glass plate is placed in the path of the ray from S₁ to P.

The path difference between the rays in absence of glass plate is

In presence of the glass plate, the optical path length of the ray from ,S₁ to P will be different. The total optical path length for this ray is given by

Where p is the refractive index of the glass plate and t is its thickness. 

Hence, the new path difference is given by

and y = y_n distance of the nth bright fringe from the central fringe

Hence, the fringe width remains constant.