An image formed when the atmosphere behaves as a lens. Mirages have a very small angular extent compared with that of the sky: They are normally seen near the horizon and involve image displacements and distortions of less than half a degree. Consequently, even though visible with the naked eye, they are easiest to see with the aid of binoculars or when photographed with a telephoto lens. The name applied to a particular type of mirage is dependent upon the way in which the appearance of the image differs from that of the object. The simplest distinction for the observer is that between a mirage that exhibits but a single image and one showing multiple images. If there is only a single image, and if that image is displaced down from the position of the object, it is said that there is sinking; if up, looming. If the image appears vertically enlarged, there is towering; if vertically shrunken, there is stooping. Recognition of these states depends critically on one's knowledge or memory of the appearance of the scene in the absence of a mirage, because all that is seen is the image. However, the change is often so striking as to make classification fairly easy. Mirages are explained by the refraction of light through an atmosphere with a gradient of refractive index. The refractive index of air depends mainly on the molecular number density of air, but as the layer through which the majority of the refractive bending occurs is often fairly thin, this density variation is primarily dependent upon temperature. Indeed, it a simple matter to associate a particular type of mirage with the shape of a temperature profile. Because the observer is located inside this atmospheric lens, the mirage can change its appearance markedly as a result of slight changes in position, say changing the height of the observer above a surface. See fata bromosa, fata morgana.