When a car is about to stop, the speed of its wheel reduces. Why do we see or feel that the wheel of the car is moving in the opposite direction? This can also be observed of a fan at home. So why is it?
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asked Oct 17, 2012 at 11:58
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$\begingroup$ Possible duplicate: physics.stackexchange.com/q/32263/2451 $\endgroup$– Qmechanic ♦Commented Oct 17, 2012 at 19:14
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$\begingroup$ yes it's possible, but i didn't found that question when posting mine because it's not properly asked. Also i am more specific and have done more study before asking the question because he has not mentioned that the above feeling comes only when speed of the car reduces. $\endgroup$– android developerCommented Oct 19, 2012 at 13:48
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2 Answers
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Apparent reversal of rotation is a kind of visual interference effect where the frequency of the imaging system (film, tv but not human eyes) is close to the rotational frequency of spokes in the wheel. This is termed the Wagon wheel effect
In your home you have artificial light that flickers at a frequency determined by the electrical supply (usually 50 or 60 Hz) or by the lighting system. This is noticeable in fluorescent tubes suchas low-energy lighting and not really noticeable in incandescent bulbs (the filaments don't drop much in temperature in the 10 ms between voltage peaks)
This stroboscopic light-source flicker could interact with the movement of radial fan blades behind a largely radial fan grill to produce the wagon-wheel effect as the blades slow when power is removed.
The Wikipedia article referred to above identifies some unproven hypotheses about subjective stroboscopy in the human visual perception system as a whole. This effect may also be a result of "hallucinogen persisting perception disorder" in people who have used drugs like LSD.
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$\begingroup$ Thanks you very much for your answer. Though I didn't get all the things mentioned in your answer now, but i will definitely read more about it on net. Once again Thanks $\endgroup$ Commented Oct 18, 2012 at 5:17
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I think that the reason for that is related to the fixed framerate of our eye which is 24 frames per second. In each next frame you'll see the wheel turned by some angle relative to the previous frame. If this difference is from 0 to pi radians (0 to 180 degrees) you'll see it in one direction. If during each frame, the angle will be from pi to 2pi radians (180 to 360 degrees) then you'll see it rotated in opposite direction in each frame. That's why the wheels of the car go through at least 3 stages of illusion: rotating in clockwise direction, slowing down up to non-rotating state and then rotate in opposite direction, while the car accelerates or stops. But in reality of course the rotation rate of the wheels in increasing or decreasing constantly.
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4$\begingroup$ Do you have a reference to the 24 Hz framerate? $\endgroup$– BernhardCommented Oct 17, 2012 at 12:25
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1$\begingroup$ Well, it looks like I had the wrong information. At least this article (amo.net/NT/02-21-01FPS.html) states the framerate of ≥200 fps for a human eye. So probably I was wrong about the value of framerate. Thanks for paying my attention to that. $\endgroup$– BartoNazCommented Oct 17, 2012 at 12:38
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1$\begingroup$ 24 fps eyesight would cause serious problems :) have you ever seen a computer screen acting wonky on tv? Imagine seeing every computer screen (that didn't have a refresh rate that was a multiple of 24) like that.. always :D $\endgroup$– st3innCommented Oct 17, 2012 at 22:28
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7$\begingroup$ This is completely wrong. Human vision does not have a "frame rate" in the same way a video camera does. There are no discrete sampling intervals, and your perception is heavily shaped by neurology rather than the properties of your eye as it compares to a camera. $\endgroup$– Colin KCommented Oct 18, 2012 at 2:18
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1$\begingroup$ @BartoNaz: That article linked in your comment is about what frequency of flickering we can perceive, not about a frequency at which the eye operates. Our ability to perceive flickering is limited by an effect called "persistence of vision" caused by the time our retina's individual photoreceptors are asynchronously "active" following photon reception. $\endgroup$ Commented Oct 18, 2012 at 10:15