What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm ? Is the system a converging or a diverging lens ? Ignore thickness of the lenses.

Explanation

Given `f_(1)=30 cm, f_(2)=-20cm, f=?`
`f=(f_(1)f_(2))/(f_(1)+f_(2))`
`f=(30xx(-20))/(30-20)=-60cm`
Thus the system is a diverging lens of focal length 60 cm.