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 object and screen, d = 90 cm. distance between two different locations of convex lens, x = 20 cm.

 As focal length,

f = (d² - x²)/4d = (90² - 20²)/(4 x 90) = ((90 + 20)(90 - 20))/360

= (110 x 70)/360 = 21.4cm.