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. 10.5 cm
B. 10 cm
C. 5.5 cm
D. 5 cm

Answer

B

Explanation

`(1)/(f) = (2)/(f_(lens)) + (1)/(f_("mirror"))`
`rArr (1)/(f) = (2)/(f_(lens))`
`because F_(lens) = (R )/(n - 1) = 2 `
`therefore " " f = (20)/(2) = 10 `cm.