Mirage is the image of an object in the flat ground surface

Abstract

It is generally accepted that mirage is formed when temperature of the ground surface, in a flat area like desert, is higher than the temperatures of the over ground air layers. In this case, light emerging from a distant object makes total internal reflection in the air layers and forms the image of the object that is called mirage. Our investigation on mirage formation in desert indicates that there is no meaningful relation between mirage formation and temperature change over the ground. In addition, we show that, the interference of the lights reflected from different air layers destroys the coherency of the image forming light. This happens because the temperature change occurs in an interval larger than a wavelength. In the second part of the report we demonstrate theoretically and experimentally that flat rough surfaces behave like mirrors at very large incident angles. We show that there is a threshold incident angle for observation of image in a rough surface that depends on the surface roughness and light wavelength. The shortest distance between observer and the image is determined by the threshold incident angle. Mirage is such an image. Image formation is studied in rough sheet glass surfaces that prepared by grinding with powders of different sizes.