Explain mirage effect

Mirage is an optical illusion caused due to total internal reflection of light in sandy deserts or in some extended surface like a black tarred road in very hot weather.

In hot summer days sandy land becomes very hot during the day time. The air in contact with the ground becomes hot and its density is reduced whereas the density of air at higher level remains unaffected.  Therefore,  the layers of air near the ground are warmer than the air at the upper level.  Thus the successive upper layers are denser than those below them.

When the ray of light from the top of a tree travels from a denser to a rarer layer, it bends away from a normal and thereby at a stage the angle of incidence becomes greater than the critical angle, the rays suffer total internal reflection and reach the eyes of the observer.  Observer sees an inverted image of the tree and concludes that there is water near the tree.