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
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Its 90 degree..... coz angle of reflection is equal ang le of incidence OF PLANE MIRROR.... PLS MEN T ION IF UR MIRROR REFERRED IS OTHER TYPE