Light travels through different media such as air, glass or water at different speeds. A fast medium is one through which it passes through more quickly than others.
- Light travels through a vacuum at 299,792 kilometres per second.
- Light travels through other media at lower speeds.
- In some cases, it travels at a speed which is near the speed of light (the speed at which light travels through a vacuum) and in other cases, it travels much more slowly.
- It is useful to know whether a medium is fast or slow to predict what will happen when light crosses the boundary between one medium and another.
- If light crosses the boundary from a medium in which it travels fast into a material in which it travels more slowly, then it will bend towards the normal.
- If light crosses the boundary from a medium in which it travels slowly into a material in which it travels more quickly, then the light ray will bend away from the normal.
- In optics, the normal is a line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.