Question

If a ray falls normally on a plane mirror than angle of ref. will be...

Answer 1

This answer is very simple.

While measuring the angle of incidence and the angle of reflection the main component is the normal. All of us know that incident ray ,reflected ray and normal lie on the same plane and a normal is a perpendicular line to the plane. As I have already said before both the angle of incidence and the angle of reflection are measured with respect to normal, and as the normal is perpendicular to the plane the incident ray would not make any angle with the normal, if it is also perpendicular to the same plane. As incident ray and reflected ray are equal the reflected ray would also not make any angle.

So, the answer to your question is zero. No angle would be made.

Answer 2

so no angle will be made