The reflection of light follows certain definite laws. A ray of light striking a reflecting surface at right angles to it is returned directly along the path it has followed in reaching the surface. When, however, a ray strikes a reflecting surface at any other angle, it is reflected at an angle in an opposite direction. The incoming ray is called the incident ray. Its direction is usually described by the angle of incidence, which is the angle that it makes with the normal, or line perpendicular to the reflecting surface at the point of reflection. The angle formed by the reflected ray and the normal is called the angle of reflection and is equal to the angle of incidence. Furthermore, the reflected ray is always in the same plane as the incident ray, and this plane is perpendicular to the surface.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.