Some rays falling on a converging lens are focussed 20 cm from the lens. When a diverging lens is placed in contact with the converging lens, the rays are focussed 30 cm from the combination. What is the focal length of the diverging lens?

Explanation

u = `-20` cm
`upsilon = 30 cm`
`(1)/(f)=(1)/(upsilon)+(1)/(u) rArr (1)/(f)=(1)/(30)-(1)/(20)`
`(1)/(f)=(20-30)/(30xx20)=(-10)/(30xx20)`
`(1)/(f)=-(1)/(60)`
`f=-60cm.`