Question

Under what condition is the first focal length of a lens not equal to its second focal length?

Answer 1

Answer:

If the medium on both sides of the lens is same, the first and second focal length are equal, i.e f1 = f2 (numerically).

Answer 2

Explanation:

If medium on both sides of a lens is same its first and second focal lengths are equal i.e, f1 = f2 (numerically)