Question

A luminous object is placed 20 cm from surface of a convex mirror and a plane mirror is set, so that virtual mirror is at a distance of 12 cm from object, then focal length of convex mirror is

• A. 5 cm
• B. 10 cm
• C. 20cm
• D. 40cm

Answer 1

Answer: (A)

Here, $ u=-20cm,\text{ }v=12+12-20=4cm $ $ \therefore $ $ \frac{1}{f}=\frac{1}{v}+\frac{1}{u}=\frac{1}{4}-\frac{1}{20} $ $ =\frac{5-1}{20} $ $ =\frac{4}{20}=\frac{1}{5} $ $ \therefore $ $ f=5\,cm $