Question

Prove that in mirror parallel ray passes through focus

Answer 1

The reflecting ray of a parallel ray to the principle axis passes through principle focus

Answer 2

Explanation:

Any ray travelling parallel to the principal axis in case of concave mirror is supposed to pass through principal focus after being reflected from mirror surface .However a ray of light parallel to principal axis and coincident with it intersects mirror surface at pole and just retraces it's path after reflection therefore satisfying the condition of being parallel to principal axis as well as being reflected at pole of mirror.