vacuum
Although the speed of light is constant in a vacuum, light travels at different speeds in different kinds of matter. For example, light travels more slowly in glass than in air. Therefore, when light passes from air to glass, it slows down. If light strikes a sheet of glass straight on, or perpendicular to the glass, it slows down but passes straight through. However, if light enters the glass at an angle other than 90, the wave refracts, or bends. This is illustrated in Figure below . How much light bends when it enters a new medium depends on how much it changes speed. The greater the change in speed, the more light bends.