Temperature profile and double images in the inferior mirage

Abstract

Desert mirages were simulated in the laboratory by heating a flat surface of sand. This showed that a boundary layer of air, of only a few millimeters immediately covering a heated surface has nearly the same thickness over a wide range of temperatures. It consists of a region producing an inverted image, and another producing an erect image depending on the relative distances between observer, heated surface and object. Measurement of distances and heights show angles of incidence greater than angles of reflection. Air refractive index measurements agree with involvement of the critical angle in ray bending. An analogy with density gradient sucrose solutions also shows double images, inverted and erect. It is assumed that a similar situation exists outdoors. Specular reflection over rough surfaces can be described as a mirage, but it is seen at a wider range of viewing angles, and it does not require a temperature gradient. It is always present, but it can be obliterated by a higher intensity of diffuse reflection.