# A question about a visual effect on a rotating prism

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.

Danger
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.

A.T.
Over I want to know the mathematical/physical explanation of this.
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:

Danger
Gold Member
A.T., you had better be prepared to provide me the solution to that... 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:

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

I am sorry for my language barriers :P