Question

A screen is placed 100 cm from the object. The image of the object on the screen is formed by a convex lens at two different locations, separated by 20 cm . Calculate the focal length of the lens used.

We get two equations: v+u=100 which is fine but how do we get v-u=20? I don't understand how two images will be formed and how to deal with such problems.
Will it always be v - u= separationor is it different for concave lens

Answer 1

We get two equations: v+u=100 which is fine but how do we get v-u=20? I don't understand how two images will be formed and how to deal with such problems. Is it always v-u= SEPARATION or is it different for concave lens.

Answer 2

Hope it will help.........