Question

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.

Answer 1

take it easy..., You can use this formula

\[\frac{ 1 }{ f } = \frac{ 1 }{ so } + \frac{ 1 }{ s' }\]

where, f is the focal length of the lens., "so" is an object is placed 90cm from a screen, and s' is is the distance the shadow of the object

Answer 2

This is called Bessel's method.