A Youngs double slit set up for interference is shifted from air to within water, then the fringe width :

Question

A Youngs double slit set up for interference is shifted from air to within water, then the fringe width : (A) becomes infinite (B) decreases (C) increases (D) remains unchanged

Tardigrade Admin · Accepted Answer

When Young's double-slit set up for interference is shifted from air to within water then the fringe width decreases because the refractive index of water is more than that of the air.
Originally the fringe width is given by:

β_{1} = \frac{λ D}{2 d}

The new fringe width within water will be given by

β_{2} = \frac{λ D}{2 n d}

So,

β_{2} = \frac{β _{1}}{n}

Here,

n

is the refractive index of medium

Q. A Youngs double slit set up for interference is shifted from air to within water, then the fringe width :

becomes infinite

decreases

increases

remains unchanged