Mirage and Total Internal Reflection

A mirage is an optical illusion in which an inverted image of a distant object is formed due to total internal reflection.
Mirages are typically observed in deserts during hot summer days.

Intense heat from desert sand heats the adjacent air layer, making it less dense than overlying layers.
Air density gradually increases with altitude, creating layers of continuously increasing density upwards.
Light rays from a distant object pass successively from denser to rarer air layers, refracting away from the normal at each boundary.
When the angle of incidence at a particular layer exceeds the critical angle, the light undergoes total internal reflection.
The reflected rays reach the observer, producing an inverted image of the object.
Continuous fluctuations in air density cause the resulting image to appear quivering and unsteady.