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

Distance between the screen and the object $D=90cm$.

Distance between two location of convex lens, $d=20cm$

The focal length of the lens is given by:

$f=\frac{D^2-d^2}{4D}$

$\Rightarrow \frac{(90{)}^2-(20{)}^2}{4\times 90}$

$\Rightarrow 21.39cm$