Why does a person sees water in the desert even then when no water is present there at that time? How does this happens?
Ideal conditions for a mirage are layers of air in contact with ground that has been heated by the sun .
As one goes up from the ground the temperature slowly decreases. this gradient of temperature changes the density of air . The layer closer to the ground is rarer and upper layers are denser .
Thus refraction of light from rarer to denser medium takes place and the light coming from sky takes a curved path to reach a person's eyes.
The illusion of water as mirage comes from the fact that the human brain assumes that light travels in a straight line.
A person looking at the road ahead on a hot, still, day will see the sky because light from the sky is taking the curved path .
The human brain interprets this as water on the road because water would reflect light from the sky in much the same way that a vertical temperature gradient in the layers of air does.
On hot sunny days even the car-drivers in the city highways see water ahead at the road (mirage) as the road gets hot and then a vertical temperature gradient (just like desert condition ) produce the above mentioned effect.