Question

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

Answer 1

Correct Answer - C
Using `1/v-1/u=1/f`, we get
`1/(+(f+40))-1/(-(f+10))=1/(+f)`
Solving this equation, we get
`f=+20 cm`