Backwards Spinning Wheels

Introduction

Television holds many mysteries. Like how long will it be before we see another Tony Danza sitcom? A listener phoned in concerning another, more scientific mystery: why do car wheels on TV sometimes seem to spin backwards?

Podcast

Backwards Spinning Wheels

Transcript

Why wheels on tv spin the wrong way. I'm Bob Hirshon and this is Science Update.

Probably while watching a car commercial, Eldridge Ward Frederick of Gaithersburg, Maryland called the Why Is It line.

Frederick:
Why is it when you see especially a spoked wheel vehicle, going a certain speed, that the wheels appear to be turning backwards?

Well, Eldridge, the illusion happens because tv cameras shoot 60 pictures per second. If the spokes of the wheel are spinning at, say, 59 turns per second, there will be a mismatch, and the wheel may seem to spin slowly backwards. To make it clearer, try imagining a giant stop watch on a stage. Let's say it has a fast-moving hand that sweeps once around the dial in one second. If you turned out all the lights and turned on a strobe light that flashed exactly once per second, the hand would look frozen, because you would only see it when it was straight up at 12 o'clock. Now, imagine you set your strobe to flash a little more quickly, so it would light up the clock before the hand reached noon. So it would flash once when the hand was at noon, then again when the hand had only reached eleven, and then when it reached just ten, and so forth. To the audience it would look like the hand was slowly moving backwards. When a wheel is spinning at just a touch less than the speed at which a television camera records it, the same thing happens.

If you've got a science question, don't spin your wheels. Call us at 1-800-WHY-ISIT. If we use it on the show you'll get a free Science Update mug.

For the American Association for the Advancement of Science, I'm Bob Hirshon.

Making Sense of the Research

You can’t believe everything you see on TV. Watching television is actually a very complicated process involving streams of electrons, small colored pixel dots, and thousands of still images. Seeing things is an even more complicated process related to the physics of light, the anatomy of the eye, and the interaction of nerves relaying information from the eye to the brain. It is the different parts of this process that camera tricks and optical illusions manipulate to fool the eye into seeing impossible, nonexistent, or multiple images. All this goes to show that what you see is not always what you get.

Now try to answer the following questions:

1. At what speed do TV cameras record pictures?
2. What happens when the speed of the spinning wheel (turns per second) is slower than the speed of the TV camera recording?
3. What would happen if the speed of the spinning wheel were faster than the speed of the TV camera?
4. What other things might look real on TV but might not actually be real?
5. Can you think of any things in real life or nature that fool the eye?

Going Further

To get a better understanding about televisions and camera tricks, go to: How Television Works and How Do Blue Screen Special Effects Work.

To see more examples of optical illusions go to: Optical Illusions and for interactive optical illusions and explanations for them go to: The Exploratorium Seeing Exhibit.

For an explanation of how visual perception is used in art go to: Vision and Art.

To read about an optical illusion in nature, go to: Scientists Offer Answer to Baffling Lunar Illusion; How Stuff Works Question of the Day; Eclipse Links; and Why Eclipses Happen.