Question

define principal focus of a concave lens...

Answer 1

A convex lens is thicker at the centre than at the edges. ... When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus. The distance between the principal focus and the centre of the lens is called the focal length.

Answer 2

Principal focus of a concave lens is the point on the principal axis where rays incident parallel to the principal axis will appear to diverge from that point when produced behind the lens, after getting refracted by the lens. In the fig. F denotes the principal focus of the lens.