What is a mirage?

Mirages are optical illusions produced by extraordinary atmospheric conditions. They can be non-existent sheets of water or similar visions, inverted or oversized images of distant objects or various other distortions. All are caused by the refraction, or bending, of light rays as they pass through two layers of air with different densities. The differences in density are usually due to unequal distributions of temperature in the atmosphere.

A common type of mirage is seen in deserts where the heat of the sand raises the temperature of the lower air to make it substantially less dense than the rest of the atmosphere. This bends the light to such an extent that images of the sky are projected on to the ground as patches of water. The phenomenon is known as an inferior image.

If the layers of hot and cold air are reversed, the result may be a superior mirage, where disturbed images of distant objects on the ground are projected into sky. In this kind of mirage there are sometimes two images, the lower and being upside down.

A famous mirage known as the Fata Morgana can be seen in the Strait of Messina, between Italy and Sicily. Here the distorted images of houses on the opposite cliffs are transformed into imaginary castles in the sea and sky. The Italians named this mirage after the Fata (for Fairy) Morgana, a legendary enchantress with the magical power of raising phantom castles from the waters.