Optical Illusion: Car Wheels Going 'Round

Professor Steven Ackerman of the University of Wisconsin explains a common optical illusion to Jacki Lyden. Why does it appear that spoked car wheels are rotating in odd directions?

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JACKI LYDEN, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Jacki Lyden.

We often say that you have to see it to believe it, but sometimes our eyes really do play tricks on us. For today's Science Out of the Box, we look at what optical illusions can teach us about how our brains work.

Our series has inspired a lot of listeners to write in. Today, we'll try to answer two questions about visual phenomena. The first is from Robert Blaskowitz(ph) in St. Louis, Missouri.

Mr. ROBERT BLASKOWITZ (Listener): I've always wondered why, when I'm watching a car accelerate, that its wheels appear to be turning in the right direction for a while but once it reaches a certain speed, the wheels begin to appear to spin backwards.

LYDEN: We'll turn this question over to Steve Ackerman from the University of Wisconsin, Madison. He's an atmospheric scientist and a contributor to the University Why Files, as in W-H-Y, a Web site that explains the science behind the news.

So Professor Ackerman, why do car tires sometimes appear to spin backwards?

Professor STEVE ACKERMAN (University of Wisconsin, Madison): It's something we call a stroboscopic effect and it's something that you commonly see in Western movies on television, so we also call it the wagon wheel effect. And it's actually just an optical illusion in which the spoke wheel just appears to rotate differently than the true rotation. And actually it may look like it's rotating faster or not at all or even in, as I was saying, the opposite direction.

And it's illusion that's very similar to what we see with camera strobing that we might see in dance club or a party, when you have a flashing light that kind of freezes dances in a series of images.

If we think of the wagon wheel and we imagine that one of the spokes is painted white and it's pointed straight up - so like at like 12:00 on a clock - if we rotate that wheel at once per second and then we take images once every second, in our minds, it's going to look like that spoke never moved, because it's always going to be pointing at the 12.

If we were to suddenly now start capturing those images faster than once a second, so faster than it's rotating, that white spoke that's pointing up at 12, the next time we see an image, it's not going to make it all the way around, so it's going to be pointing at 11. And then the next image that we take, it's going to be pointing at 10. And so we're going to imagine that it's going to be moving in this counter-clockwise direction instead of the clockwise direction.

LYDEN: So our eyes are taking pictures, in effect.

Prof. ACKERMAN: Yes. Very rapidly.

LYDEN: And then in these little frames, our brain is continuing to piece them together in a little stream.

Prof. ACKERMAN: That's the way that I think of it, yeah.

LYDEN: And in essence, the camera in our brains can't keep up with the spinning spokes, so it appears to us that the spokes are moving backwards. Well, Professor Ackerman, another mystery demystified. Thank you.

Prof. ACKERMAN: Thank you. This was a pleasure.

LYDEN: You can read other science answers by the Why Files at whyfiles.org, and you can submit your own Science Out of the Box questions on our Web site, NPR.org.

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