A Mirage is an optical illusion that can be sometimes observed on hot days.
When the earth’s surface is heated by the sun, the temperature of air increases. This produces a layer of hot air of lower mass density(mass per unit volume) and lower refractive index at the surface. Hot air works as an optically rarer medium relative to cool air. When the temperature changes rapidly in the vertical direction, as the refraction of light takes place, the angle of refraction changes continuously. The rays of light from the top of an object such as a car or tree cross the rays from the bottom of the object on their way to the observer’s eye. Hence an inverted image is formed below the objects true position and downward towards the surface in the direction of air at a higher temperature. In this case, some rays of light bend back up into the denser air. Mirage produces an impression of water near the hot ground
There are two main forms of mirage, classed according to whether the image of a distant object appears lower or higher that would normally be expected. An inferior mirage occurs when the ground surface is strongly heated and the air near the ground is much warmer than the air above. Everyone might know the this of mirage where pools of water appear to be lying on a hot road.
In a superior mirage, the opposite conditions occur: the air close to the ground surface is much colder than the air above, which is known as a temperature inversion. Light is bent downwards from the object towards the viewer so that it appears to be elevated or floating in the air. Superior mirages are less frequent than inferior ones and a more common over larger water bodies, which are sometimes much colder than the air above it, e.g. in spring. Superior mirages are also frequent in high-latitude regions, such as Island and over glaciers.
A famous superior mirage is the Fata Morgana, most frequently seen in the Strait of Messina between Italy and Sicily. However, Fata Morgana’s are also frequent in deserts, after night time radiation has cooled down the sand to temperatures lower than the air above. Distant objects appear extremely elongated, giving the impression of buildings and towns in the distance. This phenomenon is also known as ‘castles in the air’.