During the summer season, travellers in desert often get an impression of the presence of a pool of water at a distant place. But on reaching there they do not find a single drop of water. The desert deers also get the similar impression. They run here 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. It is simply an optional illusion and is called mirage. Do you know how is it caused?
The mirage is caused by total internal reflection of light. During the summer, the earth’s surface becomes very hot. These heats up the air in its vicinity but the upper layers of the atmosphere remain cool. Since hot air is always lighter than cold one, 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 enter from a medium of higher refractive index to a medium of lower refractive index, they get deviated from their path. This phenomenon is called refraction of light. As the rays travel more distance of varying refractive index the angle of refraction goes or increasing and a stage comes when it becomes more than 90°. At that stage the refracted light goes back into the medium from which it 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 formed in water. This gives rise to the illusion of the presence of a pool of water. This is what is known as a mirage.
Sometimes another kind of illusion producing the opposite effect takes place. Over water, 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.