Q 9.20 A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20cm. Determine the focal length of the lens.
As we know that the relation between focal length , the distance between screen and distance between two locations of the object is :
Given: = 90 cm., = 20 cm ,
so
Hence the focal length of the convex lens is 21.39 cm.