Mirage

Mirage, realistic image of an object that is either totally imaginary or that appears to be in a location other than the true one. The imaginary vision is a psychological aberration sometimes experienced by persons suffering from such conditions as extreme thirst, or mental or physical strain. The phenomenon that causes objects to appear out of place, usually in desert or at sea, is the result of atmospheric conditions. When heat radiates from a hot earth surface, as in a desert, it causes a diminution of the density of the air just above the surface and forces a denser layer of air to remain above the hot, rarefied air instead of, as is usually the case, below it. The boundary between the two layers produces a lenslike effect and refracts or bends rays of light from a distant object; it also gives the appearance of a layer of water (see Optics). The image produced by the rays bent by abnormal vertical distribution of air density appears inverted and below the real object, just as an image reflected in water appears when observed from a distance. A common experience of this phenomenon is the mirrored reflection of objects on a paved road in hot weather.

In the case of a mirage at sea, the denser layers of air are next to the cool surface of the water, and the reflection takes place from the rarer atmosphere above. Thus the object appears distorted, elongated, inverted, and suspended in the air, producing a so-called looming effect.

The fata morgana, which is a double mirage of the looming effect, produces exaggerated images of routine objects. This is seen most often in the Strait of Messina, Italy, and also over the Great Lakes in the United States.