There are two types of mirages: inferior and superior. Mirages are caused by light refraction, which is the bending of light waves. Refraction occurs when light goes through one type of material into another. For example it may pass through air and go into water. Light can also be refracted when it passes through an area of warm air into an area of cooler air because cold air has a higher density than warm air.
In an inferior mirage, which is the most common type, an object seems to be present as if it were both the actual object and its reflection in a pool of water. When the ground is very hot, heat radiates up out of the ground and warms the air directly above it. When light passes through the cooler air above into the warmer air below, it bends and creates a mirage. The mirage looks like an object that is reflected in water because part of the light would usually go to the ground, but it is bent and goes up to your eyes instead, creating a double image. Inferior mirages are commonly seen on paved roads during hot days, and often look like puddles of water on the road's surface. This is the mirage in the desert — the scenario most frequently associated with mirages.
A superior mirage appears above the horizon because a mass of cool air is below a mass of warm air. It usually appears over ice or very cold water. In this type of mirage, the object appears to be much higher than is logical. Examples of this type commonly seen by sailors are images of floating islands and masses of land where there are none. Superior mirages are most commonly seen in polar regions due to the large quantities of ice and very cold water.
Superior mirages are visible at long distances because the Earth is round. If the Earth were flat, the light which was bent downward would reach the ground very close to the place in which the light was bent, and the mirage would be visible only at a very close distance. An interesting example of this occurred in 1596, when a ship looking for the Northwest Passage got trapped in ice far north in the Atlantic Ocean and the crew had to stay there throughout the winter. Since the light of the sun was refracted in a curve following the curvature of the Earth, the crew saw the light of the sun two weeks before the actual end of the very long midwinter night.