You might have observed in a movie or on a television that the wheels of speeding automobiles sometimes appear turning backwards. Do you know what makes the forward-moving wheels appear going 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. The 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 per second are shown on the screen, and they appear continuous because the impression of an image lasts for 1/16th of a second on the retina of our eye.
So in this case each individual picture shows the automobile moving along the road. However, if the wheel does not make a complete turn from 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 going backwards. This is called the stroboscopic effect.