Mirage formation in deserts Why does a person sees water in the desert even then when no water is present there at that time?
How does this happens?
 A: 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.
