Question

A converging lens is kept coaxially in contact with a diverging lens – both the lenses being of equal focal length. What is the focal length of the combination?

Answer 1

The focal length of the lenses is f and −f.

We know that the equivalent focal length of a combination of lenses is

`1/f_e""=1/f_1""+1/f_2""=1/f-1/f=0`

`:.f_e = oo`