A screen is placed 100 ~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 . The focal length of the lens is

Step-by-step solution

Given, distance between screen and object, d=100 ~cm Separation between the two pośitions of convex lens, x=20 ~cm focal length of the lens, f= d^2-x^2 4 d aligned & = (100)^2-(20)^2 4 100 & = 10000-400 400 = 9600 400 & =24 ~cm aligned