Question

A luminous object is placed at a distance of 30 cm from a convex lens of focal length 20 cm. On the other side of the lens, at what distance from the lens must a convex mirror of radius of curvature 10 cm be placed in order to have an upright image of the object coincident with it ?

• A. 30 cm
• B. 40 cm
• C. 50 cm
• D. 12 cm

Answer 1

Answer: (C)

50 cm

Using the lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$u=-30cm$ $f=20$cm, gives
$v=60$ cm.
For coincidence this image should form at the centre of curvature of the mirror,
Since the radius of curvature is 10, the distance between the lens and the mirror will be $60-10=50$ cm