Angle of incidence and angle of reflection are the two angles formed when light reflects off a mirror. The angle of incidence is the angle at which the light hits the mirror, and the angle of reflection is the angle between the normal and the reflected ray. The two angles are equal in magnitude but in opposite directions (if one is 20 degrees, the other is -20 degrees). The reason for this is that the mirror acts as a perfect reflector; it reflects light rays in the direction normal to it, regardless of the angle at which they strike it.