Question

The distance between object and its 3 times magnified virtual image as produced by a convex lens is 20 cm. The focal length of the lens used is _____ cm.

Answer 1

Correct answer is : 15

v = 3u 

v – u = 20 cm 

2u = 20 cm 

u = 10 cm

\(\frac {1}{(-30)}-\frac {1}{(-10)}=\frac {1}{f}\)

f = 15 cm