Mirage

Mirage, pronounced muh RAHZH, is a type of optical illusion. A mirage may occur when a person is driving and sees what seems to be a pool of water lying on a hot paved road ahead. But when the person reaches the spot, the water has disappeared or has seemingly moved farther down the road. Mirages may include distant objects that seem to be closer than they truly are. Other objects, such as a mountain or a ship, may seem to float in the sky. Mirages can be seen in deserts, at sea, or in the Arctic.

A mirage can be caused by light rays bending when they pass through substances of different densities. In this drawing, light rays from the distant rocks bend as they pass from the cool, heavy air near the surface to the upper warm, light air. This produces a mirage that makes the rocks appear closer than they are.
From The World Book^™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved.
World Book illustration by Sarah Woodward.

The bending of light rays as they pass through air with different temperatures causes a mirage. This bending motion is called refraction (see Refraction). The most common type of mirage is the oasis mirage. Desert travelers sometimes experience it when they see a distant pool of water that appears to be an oasis. But when they reach the spot, they find only dry sand.

An oasis mirage can be produced when light from the sun heats the ground and the air just above it. A ray of light from the sky passes first through the cooler air high above the ground and then into the warmer or hotter air near the ground. The speed of the light increases as it enters the warmer air because fewer air molecules get in the way of the light. The gradual change in the speed of the light forces the ray to change its direction. The ray bends upward just before it reaches the ground.

A person cannot see the path the ray takes. Since the ray is traveling upward when the person sees it, the person thinks that the ray came from a spot on the ground. Hot air above the ground mixes rapidly with other layers of air heated by the sun. As the ray travels through these layers, the hot air causes the ray to ripple or "wiggle." The light of an oasis mirage is blue because it comes from the sky. Therefore, the person concludes that this blue spot must be a pool of water that has small waves. But what the person really sees is part of the sky. The rays that produce mirages are real. Therefore, mirages can be photographed.

Another type of mirage is known as the Fata Morgana. Fata Morganas are the most beautiful of all mirages. A Fata Morgana occurs when a layer of hot air traps rays of light coming from the distant objects. Objects such as rocks or chunks of ice appear to be towers of a fairy-tale castle.

Contributor: Jearl Walker, Ph.D., Prof. of Physics, Cleveland State Univ.

Mirage

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