When a pin of height 4 cm is fixed at 10 cm from a convex lens, the height of the virtual image formed is 16 cm . Find the focal length of the lens.

Explanation

Data : Convex lens, Convex lens , `h_(1)=4 cm , `
`h_(2)=16 cm` (virtual image ) , `u=-10 cm, f=?`
`(h_(2))/(h_(1))=v/u`
`therefore (16 cm)/(4 cm) =(v)/(-10cm)=(1)/(10cm)-(1)/(40cm)`
`=(4-1)/(40cm) =(3)/(40 cm)`
`therefore f=(40)/(3)` cm =13.33 cm(approximately)