Question

The plane surface of a plano-convex lens of focal length f is silvered. It will behave as:

• A. plane mirror
• B. convex mirror of focal length 2f
• C. concave mirror of focal length f/2
• D. none of these

Answer 1

The correct option is C concave mirror of focal length f/2

Focal length of effective lens

$\frac{1}{F}=\frac{2}{f_l}+\frac{1}{f_m}=\frac{2}{f_l}+\frac{1}{\infty }\Rightarrow F=\frac{f_l}{2}$