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A question about a visual effect on a rotating prism

  1. Nov 29, 2014 #1
    Hello folks,

    I was playing with a rectangular prism 4x4x3 (almost a cube) and I was giving him a rotating movement with the hand over the table so I could see a visual effect I have seen many times in my life and now I remembered to ask about it.

    So when I give to it an impulse and it start to rotate I can see very well defined smooth edges, well really a circular edge, with the actual vertex diffusal. In other words, like if it was a cylinder rotating with almost invisible vertex and edges.

    I understand that we see the only constant points in the rotation move, this is the incircle (or in-crylinder) of the prism. The vertex only passes in discrete times over the same points so that is why it becomes diffusal.

    However I want to know the mathematical/physical explanation of this.

    Thank you in advance.
  2. jcsd
  3. Nov 29, 2014 #2


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    Gold Member

    I'm not quite sure what you mean. If I understand you correctly, what you are asking about is a function of the physiology of the human visual system rather than an intrinsic property of the prism.
  4. Nov 29, 2014 #3


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    Science Advisor

    Haven't you explained it already? What exactly is still unclear? You see the cylinder where there always is a part of the spinning prism. The rest is a mix of prism and background.

    Here is a related puzzle, based on spinning stuff fast:

  5. Nov 29, 2014 #4


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    Gold Member

    A.T., you had better be prepared to provide me the solution to that... o_O
  6. Nov 29, 2014 #5

    I suppose... but only in a very lay way... There must be mathematics behind... :/

    I am sorry for my language barriers :P
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