Question

If a concave and convex lens of equal focus are kept in contact what is the effective focal length?

Answer 1

Effective focal length in `oo`.

Explanation:

When two lenses are kept in contact, powers of the lenses are simply added.

Power of a lens is positive for convex lens and negative for concave lens.

Numerically power of a lens is given by `100/f` were `f` is focal length in centimeters.

As numerically, focal length is same but opposite in size, combined power will be `Zero` i.e. focal length is `oo` i.e. it is just equivalent to plane glass,