A converging beam of rays in incident on a diverging lens. Having passed through the lens the rays intersect at a point `15 cm` from the lens. If the lens is removed, the point where the rays meet, move `5 cm` closer to the mounting that holds the lens. Find the focal length of the lens.

Options

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

Answer

C

Explanation

Given `u=10cm, v=15cm`
`(1)/(f)=(1)/(v)-(1)/(u)implies(1)/(f)=(1)/(15)-(1)/(10)`
`(1)/(f)=(10-15)/(150)implies(1)/(f)=(-5)/(150)`
`f=-30cm`