Question

define the principal focus of a concave mirror

Answer 1

A point on the principal axis of a spherical mirror where the rays of light parallel to the principal axis meet or appear to meet after reflection from the spherical mirror is called principal focus of concave mirror

Answer 2

For a concave mirror. The rays traveling parallel to the principal axis of a mirror after reflection pass through (converge at) a point F This point F is known as the principal focus of the concave mirror. It is a real point in front of a mirror.