An object 50 cm tall is placed on the principal axis of a convex lens. Its 20 cm tall image is formed on the screen placed at a distance of 10 cm from the lens. Calculate the focal lengths of the lens.

Explanation

`h_(0)=50 cm, h_(1)=2cm, v`
using, `m=(h_(1))/(h_(0))=(v)/(u)`, We get
with sign convention
`u=v(h_(0))/(h_(1))=10xx(50)/((-20))`
`=-25cm`
using, `(1)/(f)=(1)/(v)-(1)/(u)`, we get
`(1)/(f)=(1)/(10)-(1)/(-25)=(25+10)/(25)`
`f=(250)/(35)=7.14 cm`.