Mirage
Mirage, an optical illusion caused by light that is refracted (bent) as it passes through layers of air of different temperatures. Mirages are most commonly seen on deserts, on hot highways, and over large bodies of waterplaces where the temperature of the air varies markedly within a relatively short distance above the surface. Mirages can be photographed; they are not hallucinations and are not caused by the viewer's thirst or tiredness. They are images of real objects, although not necessarily the objects they seem to be.
The temperature of the air affects the air's density; in general, warm air is less dense than cool air at the same pressure. As light passes through layers of air of different densities, its speed changes and it is refracted in much the same way that light is refracted by glass. Because the density of the air varies continuously, the light is not bent at a single point, but from point to point along a smooth curve. The refraction of light causes objects to appear above or below where they actually exist because the mind interprets the curved light rays from an object as being straight.
An inferior mirage appears below the real object, and is always an inverted (upside-down) image. Shiny patches that appear on heated highways are inferior mirages. These mirages occur when the air near the surface is much warmer than the air directly above. The light rays are bent upward so that the sky can be seen on the road. The image thus produced looks like a pool of water. Mirages often seem to shimmer, creating an illusion of rippling water. "Lakes" that appear in a desert are produced in this way. When approached, these mirages recede or disappear.
A superior mirage appears above the real object, and can be either an inverted or a right-side-up image, depending on the atmospheric conditions. Superior mirages are common above bodies of water where the air nearest the water is much cooler than the air directly above. The light rays are bent downward as they travel from the cooler air into the warmer air. The bending causes distant objects, even some that are beyond the horizon, to appear in the sky above the water. Ships, distant shorelines, and occasionally an entire horizon appear above the water.
The fata morgana is a complex and often spectacular mirage that is sometimes seen over lakes, bays, and other protected bodies of water. Its name is Italian for Morgan le Fay, sorceress in the King Arthur legends. Unlike inferior or superior mirages, the fata morgana is not an image of the sky or distant objects, but rather a highly distorted image of the water beneath it. Strange and often quickly changing shapes appear, usually resembling castle ramparts, aqueducts, forests, or even mountain ranges.