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  1. Feb 16, 2006 #1
    I don't know if this is the right place to post this question, but you're a clever lot so i'm sure you'll know the answer.

    We are all familiar with the illusion of the wheels on a car on television appearing to spin backwards when the car is travelling forward, this being caused by a mismatch of the picture frequency and the wheel's rotational speed.

    However this phenomenon can also be observed in the real world, say, when you are looking at the wheel of the car beside you as you drive down the road.

    Anyone know what causes this ?
  2. jcsd
  3. Feb 16, 2006 #2


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    First, let's rule out artificial light sources. Was the light from streetlights a factor? That would cause the same shutter effect, even if it were only partially contributing to illumination.

    I would secondly suspect the vibrations in your own car taking the role of "shutter".

    Your eyes are continually jiggling - an involuntary reflex that your brain compensates for by subtracting this effect from the image. But if something provides an artificial jiggle, your brain can't manage to subtract the effect, and this will mimic a shutter - not enough to be noticeable, except under certain circumstances.

    Try this: walk heavily in a room with a computer monitor. You may notice the screen going choppy briefly with each step. The quick vibration transmitted through your body by the impact with the ground jostles the image that your eye sees, briefly disrupting the otherwise smooth "movie" you see.

    The walking thing is a real phenomenon, but I am only hypothesizing it as the expalnation of what you witnessed.
  4. Feb 17, 2006 #3
    Sorry, I should have said that it was daylight with no artificial light.
    I have experienced the effect you talk about but am not sure that it would be regular enough to explain what was the illusion of constant backward rotation.
    I had wondered if the brain processes visual information in discreet time intervals (at a higher frequency than television ?) and so might be directly responsible ?
  5. Feb 17, 2006 #4
    Thats a really interesting question. In my attempts to google for an answer, I came up with nothing. Perhaps you should post it to the Mind and Brain forum if no one here can answer it.
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