What is a Mirage?

When light passes from one substance to another, it may also change direction.

When light rays pass from air to water or from water to air, they bend as they pass through the surface of the water. 

This bending is called refraction.

Light rays pass through other substances, as well as air and water. They travel at different speeds as they pass through these substances. As rays pass out of one substance and into another, they are refracted.

The amount that light is refracted depends on two things. 

The first is the color of the light. Red light bends less than other colors, and violet light bends more.
The second is the angle at which the light reaches the surface of the second substance. This is called the angle of incidence.
The angle at which the light leaves the second substance is called the angle of refraction.
The photograph shows a mirage in the desert in Tunisia. The heat makes light rays refract as they pass from cool air down to the hot air near the ground. This makes you think there is a cloud on the sand, but it's not really there. It's a mirage! The man in the diagram thinks that he can see a cloud. But rays from the cloud above are creating a mirage on the sand in front of him.