When a vehicle with spoked wheels is filmed the wheels often have the optical illusion that they are moving backwards. Why?
#128747. Asked by unclerick. (Jan 03 13 12:45 PM)
Apparently, this is not yet definitively explained, but: "There are two broad theories for the wagon-wheel effect under truly continuous illumination. The first is that human visual perception takes a series of still frames of the visual scene and that movement is perceived much like a movie. The second is Schouten's theory: that moving images are processed by visual detectors sensitive to the true motion and also by detectors sensitive to opposite motion from temporal aliasing. There is evidence for both theories; as of 2011, the debate is not completely settled although the weight of evidence favours the latter." -from http://en.wikipedia.org/wiki/Wagon-wheel_effect|
A good description of the former (visual frame) theory is here:
"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." -from http://www.npr.org/templates/story/story.php?storyId=6668533