Light travels through different media such as air, glass or water at different speeds. A slow medium is one through which it passes more slowly.
- Light travels through a vacuum at 299,792 kilometres per second.
- Light travels through other media at lower speeds. In some media, it travels at a speed which is nearer the speed of light as it passes through a vacuum and in others 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.