Question

Principal focus of alum or principle focus of lens is same

Answer 1

no because it is a point on the principal axis where beam of light is
parallel to principal axis

Answer 2

Answer:

The principal focus of a convex lens is defined as the point on which the principal axis of the light is parallel to the principal axis that converges after the refraction from the lens. The principal focus of concave lens is when the light rays get diverged and appear to form a single point.