Physics Question on Ray optics and optical instruments

Question

A plano-convex lens $(f = 20 cm)$ is silvered at plane surface. Now focal length will be

Answer 1

Solution

The focal length of the plano-convex lens is 20 cm with the plane surface being silvered.
So, the net power of the lens is given by
$P = \frac{1}{f} \times 2 = \frac{2}{f}$
Thus, the net focal length is $\frac{f}{2}=\frac{20}{2}=10\, cm$