A concave lens forms the image of an object such that the distance between the object and image is `10 cm` and the magnification produced is `1//4`. The focal length of the lens will be

Options

A. `8.6 cm`
B. `6.2 cm`
C. `10 cm`
D. `4.4 cm`

Answer

D

Explanation

Concave lens forms the virtual image of a real object.
So let `u=-4x `and `v=-x`
then `3x=10cm`
or `x=(10)/(3)cm`
`:. u=-(40)/(3)cm `and `v=-(10)/(3)cm`
Substituting in `(1)/(f)=(-3)/(10)+(3)/(40)`
or `f=-(40)/(9)` or `f=-4.4 cm`