Question

A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 30 cm. Determine the focal length of the lens.

Answer 1

v + u = 90;

v - u = 30;

hence v = 60 and u = 30;

Substitute u and v in lens formula to get focal length of lens = 20 cm