A mirage is an optical phenomenon that occurs most often in the desert or at sea. The
image of an object appears displaced above, below, or to one side of its true position
as a result of spatial variation of the index of refraction of air.
Desert travelers have often been deceived by the vision of an oasis and water in the
distance. However, this trick of the eye is nothing more than an unfortunate illusion.
This can also happen over water. A person watching people on the other side of a body
of water may be startled to see them walking on water. This is only a vision created by
the atmosphere. Temperature differences in adjacent air layers can cause major changes
in density. The atmosphere then acts like a lens, bending light from its usually
uninterrupted course. The human brain, however, assumes that the line of every image
seen through the eye has traveled in a straight line, even when it is distorted or
misplaced.
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