Question

A plano-convex lens of focal length $20$ cm silvered at the plane surface will behave as a convergent mirror of focal length ______ $cm$.

Answer 1

Since net power of a silvered lens is

$\frac{1}{7}=\frac{-2}{f_{lens}}+\frac{1}{f_{mirror}}$

hence according to question

$\frac{1}{7}=\frac{-2}{20}=\frac{1}{20}=\frac{-1}{20}$

$f=-20cm$