A point object is placed at a diatance of 25 cm from a convex lens of focal length 20 cm. 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. 5 cm
B. 10 cm
C. 15 cm
D. 20 cm

Answer

C

Explanation

To solve this problem first find the object for lens. This object is the image formed by the glass slab.

Applying lens formula
` ( 1 ) /( v ) - ( 1 ) /( u ) = ( 1 ) /( f ) rArr ( 1 ) /( oo) - (1 ) /(20) = ( 1 ) /( f ) rArr u = - 20 `cm
Here -ve sign indicates that object is at the left of the lens.
` therefore ` shift of image by the glass slab = 25 - 20 = 5cm
shift of image by the galss slab = ` t (1- ( 1 ) /( mu )) `
` rArr 5 = t (1 - ( 1 ) /( 1.5) ) rArr t = 15 cm `