All electromagnetic waves travel at the same speed in a vacuum, about 3 × 108 m/s. In a substance such as glass or water they travel at a different speed. In glass and water, light travels more slowly than it does in air.
When a wave enters a different substance at an angle, the change in speed makes it change direction. This is refraction. Light going from air into glass slows down and bends towards the normal, the line at right angles to the surface. Light leaving the glass into air speeds up and bends away from the normal. If a wave meets the boundary along the normal, its speed changes but it does not change direction.
The frequency of a wave stays the same when it crosses a boundary, so a change in speed means a change in wavelength. The speed of light in a substance can also depend on its wavelength. In glass, violet light is slowed more than red light, so violet is refracted more. This is why a glass prism splits white light into a spectrum.
