Mirages are not optical illusions, as many people think. They are real phenomena of atmospheric optics caused by strong ray-bending in layers with steep thermal gradients. Because mirages are real physical phenomena they can be photographed. Optical illusions, on the other hand, are quirks of human vision in which the observer sees something that does not exist physically. Of course, the distorted images produced by mirages may elicit optical illusions, when an observer misinterprets the scene, hence the confusion of these distinctly different classes of phenomena.
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 density. (Refraction also differs for the various colours (wavelengths) of the visible spectrum and is part of the process causing many atmospheric optical phenomena including rainbows.).
In a mirage, there is at least one inverted image of some object. This "mirror image" is the origin of the French word mirage, which comes from the phrase se mirer, "to be reflected; to see one's image in a mirror." .
Mirages are distinguished from other refraction phenomena such as towering (exaggerated vertical size of images), sinking (disappearance below the horizon of objects usually seen), and stooping (images squashed together vertically), in which an object may appear distorted, but not inverted.