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 1/16th of a second on the retina of our eye. This phenomenon of the image remaining on the retina for 1/16th 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.