Mirage

A mirage is an optical phenomenon involving the path of light that occurs when light rays refract as they pass through layers of air of different temperatures. Because air density depends on temperature, light does not travel in a straight line through each layer; instead, its path bends. The human eye interprets the image as coming from a location other than where the object actually is.

A mirage is not a hallucination seen only by an individual; it is a real image that can be photographed by a camera. This distinguishes it from dreams and psychological illusions. What is typically observed is a shimmering appearance in the sky, bodies of water, or an object appearing inverted or completely reversed.

How it occurs

When the ground is extremely hot—for example, sand or a dark road in intense midday sunlight—the layer of air near the ground becomes hotter than the air above it. Hot air has lower density. Light rays traveling downward refract gradually and curve back upward before reaching the ground. The eye then sees an image appearing to come from the ground, which is often actually light from the sky—which is why the ground appears to be covered with water.

Types

Mirages are generally divided into two types. The inferior mirage is one where the image appears below the actual object, such as what is seen on roads and in deserts. The superior mirage occurs when the air layer near the ground is colder than that above it—such as over icy seas—where ships or islands appear to be floating above the horizon, and sometimes appear completely inverted. There also exist complex mirages involving multiple air layers that occupy an intermediate position.

Connection to optical phenomena

The refraction of light through air also causes the horizon to appear farther away than it actually is and allows the sun to still be seen after it has actually set. A mirage is part of the same family of phenomena, but differs because it creates a distinct image of an object rather than only a slight change in position.

Significance

Mirages have an effect on marine and atmospheric operations, since they can distort the measurement of distance or the location of landmarks. They are also part of physics experiments used to explain changes in the refractive index. In travel literature and desert tales, mirages have become a symbol of something desired but unreachable.