Question

What do you understand by focal plane of a lens?

Answer 1

Focal plane of a lens: Rays of light can pass through the lens in any direction and hence there will be two focal planes on either side of the lens and they are referred to as the first focal plane and the second focal plane of a lens:
(i) First focal plane: It is the plane passing through the first focal point and normal to the principal axis of the lens.
(ii) Second focal plane: It is the plane passing through the second focal point and normal to the principal axis of the lens.