You must have seen it in movies when a person has been traveling in the desert for many days. He is suddenly delighted to see a pool of water in the distance. When he goes closer he realism that there is no water at all. What he saw was a mirage. The desert deer also gets a similar impression. They run hear and there in search of this illusory water and finally die of thirst. A similar phenomenon is observed when we drive on a concrete road on a hot summer day. Mirage is simply an optical illusion. Do you know how it is caused?
Mirage of a tree in the desert
This optical illusion is caused by total internal reflection of light. During the summer, the earth’s surface becomes very hot. These heats up the sir in its vicinity but the upper layers of the atmosphere remain cool. Since hot air is always lighter than cold air, the air near the earth’s surface becomes lighter than the air of the upper layers. In scientific terms, the refractive index of hot air is less than that of cold air. As such, the refractive index goes on increasing as we go above the earth’s surface up to a certain height.
In such circumstances, when the light rays coming from trees and plants enters from a medium of higher refractive index to a medium of lower refractive index, they get deviated form their path. This phenomenon is called refraction of light. As the rays travel more distance of varying refractive index, the angle of refraction goes on increasing and a stage comes when it becomes more than 90 degrees. At that stage the refracted light goes back into the medium from which is had started. This is called the total internal reflection of light.
As a result of this reflection, the images of the trees and plants appear inverted, giving an impression as if these images have been reflected by water. This gives rise to the illusion of the presence of pool of water. This is what is known as a mirage.
Sometimes another kind of illusion producing the opposite effect takes place. Over cold artic waters, a layer of cold air may lie under a layer of warm air. This causes the light to bend downward. For example a ship beyond the horizon may appear to be lifted into the sky. This optical illusion is known as looming.
