Question

A plano convex lens has focal length `f = 20 cm`. If its plane surface is silvered, then new focal length will be
A. `20 cm`
B. 40 cm
C. 30 cm
D. 10 cm

Answer 1

Correct Answer - D
`1/(f) = (2)/(f_(L)) + (1)/(f_(M))`
For a plane mirror, `f_(M) = oo`
`:. 1/f = 2/(f_(L))` or `f = (f_(L))/(2) = (20 cm)/(2) = 10 cm`