Distance of an object from the first focus of an equi-convex lens is `10 cm` and the distance of its real image from second focus is `40 cm.` The focal length of the lens is

Options

A. `25 cm`
B. `10 cm`
C. `20cm`
D. `40 cm`

Answer

C

Explanation

In the figure
`u=(-10+f)`
`v=(40+f)`
and `f=+f`
Using `( 1)/(v)-(1)/(u)=(1)/(f)` we have
`(1)/(40+f)+(1)/(10+f)=(1)/(f)`
Solvinig this we get
`f=20 cm`