What Causes a Mirage?
Desert travelers often tell of seeing distant pools of water, only to have the water disappear when they drew near. Those vanishing images are called mirages.
Mirages happen when the hot desert sand heats the air just above the ground. This layer of hot air reflects the light coming from the sky, just as a mirror does. The traveler sees a reflection of the sky, and the reflection looks like water.
By bending light rays, layers of cool and warm air can also produce mirages that make distant buildings and other far away objects look closer than they actually are.
Mirages are optical illusions that results from the way in which light is refracted (bent) through air at different temperatures.
Cold air is denser than warm air, and therefore has a greater refractive index. This means that as light passes down from cool to hot air, it gets bent upwards towards the denser air and away from the ground.
To your eyes, these distorted rays seem to be coming from the ground, so you perceive a refracted image of the sky on the ground. This looks just like a reflection on the surface of a pool of water, which can easily cause confusion.