Question

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

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

Answer 1

Answer: (D)

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 $\frac{f}{2}=\frac{20}{2}=10cm$