# Fast medium

Light travels through different media such as air, glass or water at different speeds.  A fast medium is one through which it passes more quickly.

• 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.
• so:
• 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 an imaginary line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.

https://en.wikipedia.org/wiki/Refraction