What is a Mirage?

            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.

Why do Formed Moving Wheels of Cars in Movies Sometimes Appear to Turn Backwards?

             Have you ever taken care to notice the rotation of wheels? While seeing a movie or a television, you my notice that the wheels of speeding automobiles sometimes appear turning backwards. Do you know why forward moving wheels appear to be moving backwards?

Wheels of cars in movies sometimes appear turning backwards

            To understand this strange  optical effect, you have to understand the basic underlying principle of the motion picture. The film projected on the screen consists of a series of individual pictures. These pictures are joined together in such a manner that you can see them one after another in rapid succession. But there is a very small time, a fraction of a second, between each frame of picture when the screen is completely dark. Usually 32 pictures are shown on the screen per second, and they appear continuous because the impression of an image lasts for 1/16^th of a second on the retina of our eye. This phenomenon of the image remaining on the retina for 1/16^th of a second is called persistence of vision. Modern movies are shot at 24 frames per second to make the individual images blend smoothly into one another, when viewed.

            In the case of moving wheels, each individual picture shows the automobile moving along the road. However, if the wheel does not make a complete turn form one still picture to the next, it appears as if the wheel is moving backwards. For example, if we watch the top edge of the wheel in one picture, and if the edge does not make a complete turn in the next picture, but instead completes three-fourth of a turn, it looks as if the wheel turned one-fourth backward. Now in the third picture, with the wheel making only three-fourth of a turn, the top edge visible in the first picture is directly at the bottom. Again it appears, as if the wheel has turned one-fourth backwards. Thus the automobile continues moving forward and wheels continue to appear to be moving backwards. This is called the stroboscopic effect. This is nothing but an optical illusion.