An object is kept at a distance of `16 cm` from a thin lens and the image formed is real. If the object is kept at a distance of `6 cm` from the lens, the image formed is virtual. If the sizes of the images formed are equal, the focal length of the lens will be

Options

A. `19 cm`
B. `17 cm`
C. `21 cm`
D. `11 cm`

Answer

D

Explanation

In first case,
`u=-16 cm`, they `v=(+16n) cm`
`:. 1/(16n)-1/(-16)=1/f` …..(i)
In second case,
`u=-6 cm`, they `v=-(6n) cm`
`:. 1/(-6n)-1/(-6)=1/f` …..(ii)
Solving these two equations, we get
`f=11 cm`