Why Do Spinning Rims Appear to be Rotating Backward?
This is one of the optical illusions of everyday life that seems to fascinate all of us, regardless of age or education. As we travel down the highway we see cars with certain types of wheels or wheel covers. We know the cars are moving forward just as fast as we are and we know the wheels are turning in a forward motion. But the rims seem to be spinning backward!
What’s happening here?
We should first consider that this illusion occurs when the car is moving slowly, especially after it has just started and gains a certain speed or when it is slowing down and reaches that same speed. If we look at this car for a longer period of time, however, the rims may even seem to be just standing still or rotating in the opposite direction the car is moving.
The reason for this is both simple to understand and difficult to grasp. The simple explanation is that our eyes can follow the motion of the individual spokes or bars on the wheel or wheel cover as long as the movement is slow enough. When the wheel is rotating at the speed mentioned earlier (whatever that happens to be) we only see quick “bits” of the spokes or individual sections. They become blurred to us because our eyes and our brain cannot process the information that is being poured in.
Our brains simply receive and process the information more slowly than it actual takes place. This effect is similar to the strobe lights and stroboscopic activity. The light source has a lot to do with how our eyes and brains see and record the information. (Think of people dancing in a strobe light at the nightclub. They seem to be standing still.)
We get just enough information to know that the wheel is rotating forward but that’s about it. With this reduced amount of information the brain may be working to make sense of what the eyes see or it may catch individual spokes a split second late, time after time. This leads to the appearance of the wheel rotating backward. Of course, we have to take into account how the light strikes the wheel spokes too. At certain angles and with certain amounts of sunlight the effect is either more noticeable or less noticeable.
It’s also interesting to note that video and animation of spinning spoke wheels can enhance this “backward” effect. This not only depends on the level of light and the angle we are viewing it from, it depends on the speed of the video. With older, film cameras the backward motion could be quite obvious.
This is just another example of the old magician’s line, “The hand is quicker than the eye.” In the case of spinning wheels on a car, what is happening is definitely quicker than the eye (and the brain).