Mirages are usually associated with hot deserts. The traveller in a desert often sees what appears to be a sheet of water a short distance ahead of him. This he is never able to reach, since it is an optical illusion.
It is not necessary to make a journey to the Sahara in order to see this phenomenon. Mirages are quite common elsewhere. On a hot summer’s day the surface0 of a hot roadway in the near distance may appear wet and shiny as if after rain. Since we are accustomed to seeing the sky mirrored in the surface of still water, the natural inference made is that the road surface is wet (Fig. 23.18).
Two explanations have been suggested. Traditionally, it has been explained by the progressive refractive bending of light from the sky as it passes through successive layers of less optically dense air of increasing temperature near the hot ground until it finally meets a layer at an angle equal to the critical angle. It then undergoes total reflection.
A rather more probable explanation is given by M. Minnaert in his book entitled Light and colour in the open air (Bell). He attributes the phenomenon to the progressive bending of light as it passes through the warmer layers of decreasing refractive.