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.

Given:

Distance between the image (screen) and the object, D = 90 cm
Distance between two locations of the convex lens, d = 20 cm
Focal length of the lens = f
Focal length is related to d and D as:

f = …(1)

Where,

f = focal length

D = distance between image and the object.

d = Distance between two locations of the convex lens.

By putting the values in equation (1) we get,

⇒ f = 21.39cm
Therefore, the focal length of the convex lens is 21.39 cm.