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 20 cm. Determine the focal length of the lens.

Answer 1

Distance between the image (screen) and the object, D = 90 cm

 Distance between two locations of the convex lens, d = 20 cm 

Focal length of the lens = f 

Focal length is related to d and D as:

Therefore, the focal length of the convex lens is 21.39 cm.