Question

A burning candle whose flame is 1.5 cm tall is placed at a certain distance in front of a convex lens. An image of candle flame is received on a white screen kept behind the lens. The image of flame also measures 1.5 cm. If f is the focal length of convex lens, the candle is placed:

(a) at f
(b) between f and 2f
(c) at 2f
(d) beyond 2f

Answer 1

 at 2f

This is because a convex lens produces an image of the same size as the object when the object is placed at 2f.