When travelling in a long stretch of desert road, one may see the presence of ‘water’ on the road at a distance. However, when one travels towards it, the appearance of ‘water’ will disappear, or it may appear again at a distance. Such an optical illusion is known as a ‘mirage’ and one usually sees such appearances when the sun is blazing hot and when the terrain is rather flat. However, many do not understand why such a phenomenon take place and this article will explain the factors that give rise to a mirage, and what people actually see as ‘water’.
In order to understand that cause of a mirage, one needs to understand how the light travels through air. When a light beam enters one medium to another, such as in the case of from water in to the air, there is a slight bending of the light beam. This is called the ‘refraction’ and it can take place even within one single medium as well. For example, when considering the air closer to the earth, it may be warmer than the air, which resides at a higher level. This creates a temperature gradient each having a different ‘index of refraction’. In general, the cooler air is considered ‘dense’ and will have a higher ‘index of refraction’ than the warmer air, which is considered ‘less dense’.
Thus, when such vertical temperature gradients exist in the air, the light travels in a curved path than its usual path of a straight line as if it is entering from one medium to another. By the time it reaches the earth’s surface, the curved path gives rise to a phenomenon known as ‘internal reflection,’ where a light beam reflects completely rather than being refracted as it enters another medium. In relation to a mirage, the light gets completely reflected at the hottest layers of air closer to the ground as it tends to take the path towards more cooler air. However, another parameter known as the ‘critical angle’ will also contribute to the formation of a mirage as when the light beam enters a different layer with a smaller index of refraction, if the angle of entry exceed the ‘critical angle’, ‘internal reflection’ will take place. Thus, what is seen as a mirage is probably the image of the sky being reflected, or perhaps the image of a distance object. However, the human brain perceives such appearances as the presence of water.
Another explanation for the curved path taken by light beams is the understanding gained from how photons travel from one medium to another. Photons are the particles know to create light and is known to quantum physicians as taking the minimum time when travelling from one point to another. These particles are also know to travel faster in hot air than in the cold air. Scientists believe that the curved trajectory taken by the light is a result of taking ‘shortcuts,’ because it requires taking the shortest path at the fastest speed.
For scientists, the phenomena underlying the formation of a ‘mirage’ was rather important because it is the same principle used for some of the modern day technologies such as optic fiber, single lens reflex cameras, and binocular telescope.