# Bullseye-shaped interference pattern in seminar-room chair

During a break n a seminar today, I noticed that the chairs in front of me all had slightly transparent black mesh fabric. The backs of the chairs were in the shape of a hyperbolic paraboloid. The fabric covering it had two layers a couple of centimeters apart (it was just a fabric sheath over a frame), and the mesh was a hexagonal tiling with hole size approximately equal to thread size, both small.

I noticed that the interference between the two meshes made a bullseye pattern. The circles in the bullseye moved inwards or outwards as I moved my head, but the center of the bullseye seemed to be constantly at the critical point of the hyperbolic paraboloid (although I couldn't move my head much from side to side).

Why did this shape appear?

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You may be referring to a Moire pattern en.wikipedia.org/wiki/Moire_pattern –  Jeremy Apr 5 '13 at 0:55
Yes, that's exactly what I'm referring to. I know that interference patterns occur(although the article really helped me understand it better), but my question is, why did it have that particular shape? –  Brian Rushton Apr 5 '13 at 0:59
@Jeremy, it is amazing that you got it from just the description. –  Lord Soth Apr 5 '13 at 1:33
I honestly couldn't quite figure out what picture you have in mind. I guess it is a bit late to ask whether you happened to take a photo of the pattern? –  Willie Wong Apr 5 '13 at 15:48
@WillieWong: I didn't, but they are in the math seminar room at UPenn in the DRL (I think 4C8 or 4C28), in case anyone one here has time, lol. –  Brian Rushton Apr 5 '13 at 15:57

Apparently if you start with an aperiodic screen pattern $A$ and produce a Moiré pattern based on $A$ and a rotation and/or rescaling $A'$, you tend to observe circular patterns. See figures 2.1c and 2.1e here. In general, I think you may be able to find an answer to your question in this book.