Question

If an object is placed 21 cm from a converging lens, the image formed is slightly smaller than the object. If the object is placed 19 cm from the lens, the image formed is slightly larger than the object. The approximate focal length of the lens is :

(a) 5 cm 

(b) 10 cm 

(c) 18 cm 

(d) 20 cm

Answer 1

(b) 10 cm

The image is slightly smaller than the object when the object lies beyond 2f; and the image is slightly larger than the object when the object between f and 2f. This means that between 21 cm and 19 cm lies 2f. Out of the given options, 20 cm lies between 21 cm and 19 cm.

So, 2f = 20 cm.

f = 10 cm.