question_answer

A lens form a sharp image on a screen. On inserting a parallel-sides slab of glass between die lens and the screen it is found necessary to move the screen a distance d away from the lens in order for the image to be again sharply focused. If the refractive index of glass relative to air is u, then the thickness of the slab is

A)  \[\frac{d}{\mu }\]                          

B)         \[\frac{\mu }{d}\]

C)  \[\frac{\mu d}{\mu -1}\]              

D)         \[\frac{(\mu -1)d}{\mu }\]