Question

An object is placed at a distance of 30 cm in front of an unknown mirror. The mirror forms an image at 20 cm behind itself. Find out the focal length and type of the mirror.

• A. 20 cm, concave mirror
• B. 20 cm, convex mirror
• C. 60 cm, concave mirror
• D. 60 cm, convex mirror

Answer 1

The correct option is D 60 cm, convex mirror

Using sign conventions, we can write:
Object distance, u = -30 cm
Image distance, v = +20 cm
Using the mirror formula,
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$
$\frac{1}{f}=\frac{1}{20}+\frac{1}{-30}$
$f=60cm$
The focal length is positive; hence, it is a convex mirror.