Question

Why the angle of incidence and angle of reflection are equal?

Answer 1

1. According to Fermat's principal of least time, light adopts the path of minimum time taken in going from one point to another.
2. According to Quantum electrodynamics, in case of a perfectly elastic collision the momentum is conserved and the light chooses the path which takes the least time to reach the final point which develops a path.
3. This makes the incident angle equal to reflection angle.

where path AB is incidence ray and path BC is reflected ray.