A converging light beam incidents on diverging lens. After passing through lens, rays intersect at 15 cm behind the lens. If lens is kept away, then these rays intersect at 5 cm. Focal length of lens is ..

Options

A. `-10 cm`
B. 20 cm
C. `-30` cm
D. 5 cm

Answer

C

Explanation

According to lens equation, `1/v-1/u=1/f`
u=-10 cm , v=15 cm ,f=(?)
`(1)/(15)-(1)/(10)=1/f`
`thereforef=-(150)/(3)`
`thereforef=-30` cm