How do mirages occur?

In movies and cartoons, mirages are often depicted as something the character craves while traveling in the desert – most often, a mirage looks like a beautiful oasis which disappears as soon as the traveler reaches the spot. In reality, a mirage looks like a shallow pond or mirror.

The main factor for a mirage to appear is the collision of hot and cold layers of air. Most often it happens in deserts, when the ground is extremely hot and the air is cooler. In the same way, mirages often appear on the roads in hot areas, when the dark asphalt quickly gets very hot.

The hot surface warms a layer of air just above it. This thin layer is much hotter than the upper ones, so it also becomes less dense. Light travels through mediums of different density with different speed and changes its direction when passing from one medium to another. This phenomenon is called refraction.

We see objects because light reflects off them and enters our eyes. When the light from the sky passes into hotter air and is refracted, our brain and eyes are unaware of all the bending it did. They think this light has come from below in a straight line as it usually does (see the green line). That's why the image of the sky (or other objects) appears on the ground.