Explanation:
Normally, the light waves from sun travel straight through atmosphere to the eye. But, the light travels at the different speeds through the hot air and the cold air.
In deserts, mirages happen when ground is very hot and the air is comparatively cool. The hot ground warms the layer of the air which is just above ground. When light moves through cold air and into layer of the hot air, the light gets refracted into a U-shaped bend due to change in the speed.
The “bent light from the sky” is refracted as it passes from the cooler air into the hotter air and back up to the eye. The brain assumes that refracted light follows the straight path. As refracted light from sky is interpreted to a straight ray, it let the person see image of sky on the ground.
This is why mirage appears as blue water.
