Question

what is the angle of incident ray is at 90degree at the incident ray?

Answer 1

0°
as angle is normal

Answer 2

If the incident ray falls along the normal two things need to be considered like The angle of reflection is calculated with respective to normal,so if the ray falls along the normal then angle of reflection = angle of incidence =0 The ray reflects back without deviating Therefore the angle is 0 degrees.