Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction. We can use water waves in a ripple tank to show this effect.

Refraction doesn't happen if the waves cross the boundary at an angle of 90° (called the normal) - in this case, they carry straight on.

The refraction follows a regular pattern.

When light passes from air into a block of glass the following is seen:

Light ray hits glass block at right angles to surface. Wave slows, its wavelength decreases as it enters glass. As wave returns to air, speed and wavelength increase to original values.

The light enters the curved face of the block directly, so no refraction is seen here. As you increase the angle of incidence you see a greater angle of refraction.

At a specific angle, the light ray will no longer leave the block. At this point the angle of incidence is called the critical angle. Any further increase in the angle of incidence will mean the ray is reflected, not refracted.

When white light passes from air into a triangular prism, it is refracted as it enters, and then again as it exits. As it leaves the prism, the different wavelengths of the individual colours of light result in different angles of refraction.

This splits white light into the seven colours of the rainbow. This process is called dispersion. Red light is refracted the least and violet the most. Each colour of light can be called monochromatic.