An object kept 60 cm from a lens gives a virtual image 20 cm in front of the lens. What is the focal length of the lens? Is it a converging lens or diverging lens?

Answer

Given : Objective distance (u)=-60 cm
Image distance (v) =- 20 cm
To find: Focal length (f)=?
Formula: `(1)/(v)-(1)/(u)=1/f`

Explanation

`1/f=1/v-1/u`
`therefore 1/f=(1)/(-20)-(1)/(-60)`
`therefore 1/f= (-1)/(20)+1/60`
`therefore 1/f= (-60+20)/(20 xx 60)`
`therefore 1/f =(-40)/(20xx 60)`

`therefore 1/f =(-1)/(30)`
`therefore f=-30`
f=- 30 cm, Since f is -ve, Lens is concave i.e diverging lens.