mirage

mi·rage   (mĭ-räzh')    n.   An optical phenomenon that creates the illusion of water, often with inverted reflections of distant objects, and results from distortion of light by alternate layers of hot and cool air. Also called fata morgana. Something illusory or insubstantial. [French, from mirer, to look at, from Latin mīrārī, to wonder at, from mīrus, wonderful; see smei- in Indo-European roots.]

mirage 

Main Entry: mi·rage
Pronunciation: m&-'räzh
Function: noun
: an optical effect that is sometimes seen at sea, in the desert, or over a hotpavement, that may have the appearance of a pool of water or a mirror in which distant objects are seen inverted, and that is caused by the bending or reflection of rays of light by a layer of heatedair of varying density

mirage   (mĭ-räzh')  Pronunciation Key  (click for larger image in new window) An image formed under certain atmospheric conditions, in which objects appear to be reflected or displaced or in which nonexistent objects seem to appear. For example, the difference in the index of refraction between a low layer of very hot air and a higher level of cold air can cause light rays, travelling down from an object (such as the sky or a cloud) and passing through ever warmer air, to be refracted back up again. An observer viewing these light rays perceives them coming up off the ground, and thus sees the inverted image of the object, which appears lower than the object really is. In this way the sky itself can be reflected, resulting in the mirage of a distant lake.