Mirage: Natural and Artificial

Abstract

NATURAL mirage is due to the passage of rays of light through strata of air in which the refractive index is a function of the height of the ray above the ground. In these strata the air, though not homogenous, is isotropic, that is, its optical properties at every point are the same in all directions but vary from point to point. In natural mirage the non-homogeneous strata generally form horizontal layers, and in different conditions the air above or below the stratum may be the denser. The refractive index of a gas depend almost entirely on its density, and, hence, if the pressure is constant (which may be assumed for the thin mirage producing layers) light will travel faster as the height of the ray increases if the warm air is uppermost, and vice versa.