For a mirror, angle of incidence is equal to angle of reflection.
For a plane mirror: If the incident ray falls along the normal, the angle of incidence is 0 degrees, NOT 90 degrees. The angle is measured with the normal to the mirror and not with the plane of the mirror. Hence the angle of reflection is equal to 0 degrees. Please see the image attached for better understanding.
Bonus: For a curved mirror: Finding out the normal of a curved mirror can be tricky. The normal of a curved mirror depends on the point of incidence. A plane, tangential to the curve at that point, is drawn and a normal is drawn at the point of incidence which is perpendicular to the aforementioned plane. The angle of incidence and the angle of reflection is measured with this normal.
You can also try the activity mentioned in the reference link to understand the concept better.
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
here in the figure shown the INCIDENT ray along the normal will reflects back unlike incident ray with an angle i,reflected ray with an angle r.
When light is incident along normal, then angle of incidence is 0 degree[ Angle of incidence is angle made by incident ray with normal].
According to laws of reflection,
Angle of incidence = Angle of reflection
Therefore Angle of reflection = 0 degree