A correct definition can be found in the American Meteorological Society's Glossary of Weather and Climate. It states:

"Mirage: A refraction phenomenon wherein an image of some distant object is made to appear displaced from its true position because of large vertical density variations near the surface; the image may appear distorted, inverted, or wavering."

Mirages form when light rays emitted from a source or reflected off an object are bent as the path of the light ray crosses air layers of different densities. The technical term for this bending is refraction. The degree of bend defines the medium’s index of refractivity and depends on the medium’s dens

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