Assertion: The pattern and position of fringes always remain same even after the introduction of transparent medium in a path of one of the slit.
Reason: The central fringe is bright or dark is depend upon the initial phase difference between the two coherence sources.

Answer

B

Explanation

If a transparent medium of thickness t and refractive index `mu` is introduced in the path of one of the slits, then effective path in air is increased by an amount `(mu-1)` t due to introduction of plate. Therefore the zeroth fringe shifts to a new position where the two optical paths are equal in such case fringe width remains unchanged