Question

What happen when a ray of light falls normally on the surface of a mirror

Answer 1

If ray of light is incidence normally on a mirror, it means that it is traveling along the normal to the mirror. So the angle of incidence for such a ray of light is zero and since the angle of incidence is zero, therefore according to second law of reflection its angle of reflection will also be zero.
This means that a ray of light which is incident normally on a mirror is reflected back along the same path because the angle of incidence as well as the angle of reflection for such a ray of light is zero.

Answer 2

its called incident ray and on the angle on which is called point of incidence