A thin plano-convex lens acts like a concave mirror of radius of curvature `20 cm` when its plane surface is silvered. The radius of curvature of the curved surface if index of refraction of its matarial is `1.5` will be

Options

A. `40 cm`
B. `30 cm`
C. `10 cm`
D. `20 cm`

Answer

C

Explanation

Using the equation,
`1/F=(2(mu_2//mu_1))/(R_2)-(2(mu_2//mu_1)-1)/(R_1)`
we get
`1/(-10)=(2(1.5))/oo-1(1.5-1)/(R_1)`
Solving, we get
`R_1=10 cm`