Question

An object kept at $20cm$ from a spherical mirror gives rise to an image $15cm$ behind the mirror. The focal length of the mirror is:

• A. $-60cm$
• B. $-30cm$
• C. $90cm$
• D. $60cm$

Answer 1

The correct option is D $60cm$

Given:
Object Distance, $u=-20cm$
Image Distance, $v=+15cm$
($\because$ image is formed behind the mirror)

Using Mirror Formula,
$\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$
$\frac{1}{-20}+\frac{1}{15}=\frac{1}{f}$
$f=+60cm$
Since $f$ is positive, mirror used here is convex mirror.