The radius of curvature of curved surface of a thin plano-convex lens is `10 cm` and the refractive index is `1.5`. If the plano surface is silvered, then the focal length will be.

Options

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

Answer

D

Explanation

`(1)/(F) = (mu - 1)((1)/(R_(1)) - (1)/(R_(2)))`
`= (1.5 - 1)((1)/(oo) - (1)/(-10)) = (1)/(20)`
`F = 20 cm`
When plane surface is silvered,
`f = (F)/(2) = (20)/(2) = 10 cm`