The radius of curvature of the curved surface of a plano-convex lens is `20 cm`. If the refractive index of the material of the lens be `1.5`, it will

Options

A. act as a convex lens only for the object that lie on its curved side
B. act as a concave lens for the object that lie on its curved side
C. act as a convex lens irrespective of the side on which the object lies
D. act as a concave ens irrespective of side on which the object lies

Answer

C

Explanation

Here, `mu = 1.5`
if object lies on plane side , `R_(1) = oo, R_(2) = -20 cm`
`(1)/(f) = (mu - 1)((1)/(R_(1)) - (1)/(R_(2))) = (1.5 - 1)((1)/(20) + (1)/(oo)) = (1)/(40)`
if object lies on its curved side, `R_(1) = 20 cm, R_(2) = oo`, Fig.

`(1)/(f') = (mu - 1)((1)/(R_(1)) - (1)/(R_(2))) = (1.5 - 1)((1)/(20) - (1)/(oo)) = (1)/(40)`
`f' = 40 cm`. The lens behaves as convex. Choice ( c) is correct.