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

Here, `f_1 = 30 cm, f_2 = -20 cm, f = ?`
As `(1)/(f)=(1)/(f_1)+(1)/(f_2) :. (1)/(f)=(1)/(30)-(1)/(20)=(2 - 3)/(60) = -(1)/(60)`
`f = -60 cm`
`:.` The combination of lenses behaves as a concave lens. The system is not converging.