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mirage(mĭräzh`), atmospheric optical illusion in which an observer sees in the distance a nonexistent body of water or an image, sometimes distorted, of some object or of a complete scene. Examples of mirages are pools of water seen over hot desert sand or over hot pavement; at sea, an inverted image of a ship seen in the heavens or, also at sea, some object that is actually over the horizon but seems to loom up a relatively short distance away. These phenomena can be explained by the facts (1) that light rays undergo refraction, i.e., are bent, in passing from a medium of one density into another of different density and (2) that the boundary between two such media acts as a mirror for rays of light coming in at certain angles (see reflectionreflection,
return of a wave from a surface that it strikes into the medium through which it has traveled. The general principles governing the reflection of light and sound are similar, for both normally travel in straight lines and both are wave phenomena.
..... Click the link for more information. ). Ordinarily the density of the atmosphere gradually decreases with altitude. Variations in temperature disturb the normal state (the density of warm air is less than that of cold air), producing unusual variations in the density of the atmosphere. The "lake" mirage in the desert is essentially a reflection of the sky. Light rays coming at a grazing angle from the sky just above the horizon are thrown upward by the surface of the area of extremely hot air just above the sand, and the effect to an observer is a shimmering reflecting expanse resembling the surface of a body of water. The inverted image of a ship seen in the heavens at sea is caused by a layer of dense, cool air over the water; this layer bends the rays of light from the ship (below the horizon) in a curved path that arches over the horizon and back to earth. The image formed appears to be that produced by an object somewhere distant in a straight line from the observer and, therefore, at a position in the sky. It is sometimes inverted because in the bending process the light rays coming from the object are changed in relative position. The type of mirage described as looming, in which distant objects appear much nearer than they actually are, is explained in the same way as the image of the ship, except that the image is not inverted; the density variations may also act as a magnifying glass. Mirages can be photographed. The strange phenomenon known as the fata morgana [Ital.,=Morgan le Fay, of the Arthurian legend, the supposed author of the mirage] is a complex mirage especially in evidence at the Strait of Messina; in this mirage images of objects such as ships, houses, or men, often two of the same object with one inverted, are seen suspended in the air over the object itself or on the water.
an optical phenomenon in the atmosphère in which, in addition to a distant object (or part of the sky), a virtual image that is displaced with respect to the object can be seen. If the object is below the horizon, only the virtual image is seen. A mirage may be located below the object (inferior mirage), above the object (superior mirage), or to the side of the object (lateral mirage).
Mirages are explained by bending of rays of light from an object as a result of anomalous distribution of the index of refraction of light in the atmosphere, which is caused by the temperature distribution (and, consequently, density distribution) of the air. A superior mirage is observed above the cold surface of the earth during a temperature inversion (increased temperature with altitude); an inferior mirage is observed when there is a very high temperature gradient (that is, a sharp temperature drop with altitude) above a heated flat surface, such as a desert or a road. In such cases the virtual image of the sky creates in the process the illusion of water on the surface. For example, on a hot summer day a road receding into the distance looks wet. A lateral mirage is sometimes observed from greatly heated walls or rocks. Complex mirages with sharp distortion of the shape of objects are called fata morgana.