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 location separated by `20 cm`. Determine the focal length of the lens.

Explanation

Here, distance between object and screen, `D = 90 cm`
Distance between two location of convex lens, `d = 20 cm, f = ?`
As `f = (D^2 - D^2)/(4 D) :. f = (90^2 - 20^2)/(4 xx 90) =((90 + 20)(90 - 20))/(360) = (110 xx 7)/(36) = 21.4 cm`.