Light rays change direction when they reflect off a surface, move from one transparent medium into another, or travel through a medium whose composition is continuously changing. The law of reflection states that, on reflection from a smooth surface, the angle of the reflected ray is equal to the angle of the incident ray.

This simple concept can be easily illustrated by a flashlight and a glass mirror. In Figure 1, the visible white light emitted by the flashlight bulb is directed onto the surface of a mirror at an angle, described as the angle of incidence. This light is then reflected back into space at another angle, known as the angle of reflection.

Incandescent reflector bulbs contain a reflective coating and therefore provide directional light. The shape of the light beam is cone-like and can be specified in a range of beam spreads from narrow spot to wide flood.