When a ray of light strikes a reflecting surface its direction changes at the interface. The point at which the ray strikes a surface is known as the point of incidence. The angle at which a ray strikes a surface is known as the angle of incidence, and is measured relative to the normal, or perpendicular, to the reflecting surface. The angle of reflection is the angle between a reflected ray and the normal to the reflecting surface. The incident ray, the normal to the reflecting surface at the point of the incidence, and the reflected ray, all lie in the same plane.
Note that when the reflecting surface is rotated by an angle "theta", the reflected ray changes direction by an angle "2 x theta". In the animation below, drag the slider to rotate the reflecting surface and observe how the angle of reflection changes.