Question

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

• A. $20cm$
• B. $40cm$
• C. $30cm$
• D. $10cm$

Answer 1

The correct option is D $10cm$

The focal length of the plano-convex lens is $20cm$ 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 $\frac{f}{2}=\frac{20}{2}=10cm$