Question

When a convergent beam of light is incident on a convex mirror, the reflected rays appear to be coming from a point $20\text{ cm}$ behind the mirror and retrace their path. The focal length of the mirror is

A.
$20\text{ cm}$
B.
$10\text{ cm}$
C.
$40\text{ cm}$
D.
$30\text{ cm}$

Accepted Answer

B. Rays that retrace their path after reflection must be directed towards the Center of Curvature ($C$). Thus, the radius of curvature $R = 20	ext{ cm}$. The focal length $f = R/2 = 20/2 = 10	ext{ cm}$.

Focal length of convex mirror with convergent beam

Explanation

Practise this under board exam conditions