mirage , 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 reflection ). 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...
mirage , 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 reflection ). 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...
