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 materials at lower speeds. In some materials, it travels at a speed closer to the speed of light in a vacuum, and in others, it travels much more slowly.
- The speed of light in air is about 299,702 kilometres per second whilst it travels at approximately 123,889 kilometres per second through diamond.
- It is useful to know whether a material is fast or slow to predict what will happen when light crosses the boundary between one material and another.
- As 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.
- As 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.