For this question nishant, I am telling you the formula but you can refer D.C Pandey for it.

 

When glass plate of refractive index  is inserted then fringe shift is :

 

 

Shift,  d / D  = ( - 1)td / D      where  t = length of slab

                                                         d = Slit difference

                                                         D = Distance of screen

 

Note : this concept is not for CBSE, and for Competition you can learn the formula, its proof is not needed.