A point object is placed at a distance of 25 cm from a convex lens of focal length 20cm. If a glass slab of thickness t and refractive index 1.5 is inserted between the lens and the object, the image is formed at infinity. The thickness t is

Options

A. 10cm
B. 5cm
C. 20cm
D. 15cm

Answer

d.

Explanation

Image will be formed at infinity if object is placed at focus of the lens, i.e., at 20cm form the lens. Hence,
shift `=25-20=(1-(1)/(mu))mu`
or `5=(1-(1)/(1.5))t ` or `t=(5xx1.5)/(0.5)=15cm`