Question

define the principal focus of a concave mirror ​

Answer 1

the principal focus of concave mirror is defined as

when the light rays coming from infinity parallel to the principles axis meet or converged at a point on the principle axis after reflection from concave mirror

that point is called principal focus of concave mirror! !!

Answer 2

When the rays of light pass parralel to principal axis of concave mirror , they reflect back at front of the mirror and they converge at a point on the principal axis . These point os known as principal focus of a concave mirror.