A plano-convex lens, when silvered at its plane surface is equivalent to a concave mirror of focal length `28 cm` . When its curved surface is silvered and the plane surface not silvered, it is equivalent to a concave mirror of focal length `10 cm` then the refractive index of the mateiral of the lens is:

Options

A. 1.5
B. 55
C. 1.6
D. 1.65

Answer

B

Explanation

`1/F_(1)=(2(mu-1))/R,F_(1)=28cm`
`1/F_(2)=(2(mu-1))/R + 2/R,F_(2=10cm`