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

Answer

D

Explanation

Here, `v=+15cm,u=+(15-5)=+10cm`
According to lens formula
`(1)/(v)-(1)/(u)=(1)/(f)implies(1)/(15)-(1)/(10)=(1)/(f)impliesf=-30cm`