In optics, the deceptive appearance of a distant object caused by the bending of light rays ( refraction ) in layers of air of varying density.
Under certain conditions, such as over a stretch of pavement or desert air heated by intense sunshine, the air cools rapidly with elevation and therefore increases in density and refractive power. Sunlight reflected down from the upper portion of an object will be directed through the cool air in the normal way; although the light would not be seen ordinarily because of the angle, it curves upward after it enters the rarefied hot air near the ground, thus being refracted to the observer's eye as though it had originated below the heated surface. When the sky is the object of the mirage, the land is mistaken for a lake or sheet of water.