Question

A lens produces an image of an object on a screen. If a slab
of refractive index n is placed in between lens and screen,
the screen has to be moved by distance d behind. The
thickness of slab is
(a) nd (b)n -1/nd
(c)(n -1)d/n
(d)nd/n-1

Answer 1

When a slab of refractive index n is placed in between the lens and the screen the images shifts by a distance d

shift is given by the formula,

$d = t(1 - \frac{n \: surr}{n \: lens})$

As surrounding medium is air n surr = 1

$d = t(1 - \frac{1}{n} )$

$d = t( \frac{n - 1}{n}$)

$t = \frac{nd}{n - 1}$

the ans is d