Question

A luminous object is placed 20 cm from surface of a convex mirror and a plane mirror is set so that virtual images formed in two mirrors coincide. If plane mirror is at a distance of 12cm from object. Then focal length of convex
A. 20cm
B. 15cm
C. 10cm
D. 5cm

Answer 1

Correct Answer - d.
For convex mirror, `u+v=12xx2=24cm`
(because for plane mirror, distance of object `=` distance of image). Also, here for the convex mirror `u=20cm`, therefore `v=4cm` . Hence, using
`(1)/(v)+(1)/(u)=(1)/(f)`
We find `f=5cm`