Question

define principal focus in simple words

Answer 1

focal point- (Also called) principal focus, focus the point on the axis of a lens or mirror to which parallel rays of light converge or from which they appear to diverge after refraction or reflection.

Answer 2

The principal focus of a lens is a point on its principal axis such that if a ray of light passes througj it, (for convex lens), it becomes parallel to the principal axis after refraction.