A screen is placed `80 cm` from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by `10 cm`. Calculate the focal length of the lens used.

Answer

`19.7 cm`

Explanation

Here, `D = 80 cm, d = 10 cm, f = ?`
`f = (D^(2) - d^(2))/(4D) = (80^(2) - 10^(2))/(4 xx 80) = (6300)/(320) = 19.7 cm`