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Stange observation

  1. Dec 7, 2013 #1
    I am curious to know if anyone has any knowledge about a phenomena I observed at a local supermarket recently.
    There was a display stand base, box like in construction probably two foot x two foot square with objects in a tray on top.
    It is the base that caught my attention. All the sides were covered in small round holes close together no more than a couple of millimetres in diameter.
    If you stood away from the display stand by a about a couple of metres and looked through the holes at the front of the stand to the holes at the back. The holes at the back of the stand appeared magnified by a large percentage. They were probably one hundred times larger a couple of centimetres in diameter at least. The further you stood away from stand the larger the holes at the rear of the display base appeared. When you walked towards it the holes became smaller at the rear.
    I have not experienced this type of effect though it does seem similar to railings crossing over each other from a distance. Though this was definitely a magnifying effect which I have not seen before could anyone enlighten,thanks.
     
  2. jcsd
  3. Dec 7, 2013 #2
    It reminds me of pinhole cameras. Try googling.
     
  4. Dec 7, 2013 #3

    Baluncore

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    The holes on your side are permitting you to see different holes on the other side, but you are convinced that is one magnified hole that you are seeing.

    The arrays of holes have spatial frequencies determined by their distance from your eye. You are sampling the far array at the slightly different frequency through the near array and seeing a spatial beat frequency.
     
  5. Dec 7, 2013 #4
    Yes it could possibly be a similar effect.Have a reasonable idea as to the workings of a pinhole camera the problem would be the variabilty in the focal length.
    In other words if it were the holes at the front projecting an image onto the rear screan or vice versa you would not expect the image to appear larger or smaller depending on how far or near you were to the stand.
    The projected image should remain a constant size as the focal length within the box remains constant.Unless something weird is happening with perspective.
     
  6. Dec 7, 2013 #5

    sophiecentaur

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    Sounds to me like a version of Moire Fringes (see link). They can be seem in a two dimensional form when you look at two sets of railings on motorway foot bridges (for example) as you drive under them. It is a 'beat pattern' between two almost equal spatial frequencies (grids). The effect is also used in measuring instruments because they have the effect of magnifying small changes into large,easily observable movements of fringes. (Just amplifying on what Baluncore wrote already)
    Your case of two 2D patterns is just a slightly more complicated 2D version.

    I have also seen a similar effect with fringes formed when viewing street lights (point sources) through the nylon fabric of an umbrella at night. That is due to diffraction, I believe, rather than Moire effects.
     
  7. Dec 7, 2013 #6

    DrGreg

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    They say a picture paints 1000 words...

    attachment.php?attachmentid=64569&stc=1&d=1386440664.png
     

    Attached Files:

  8. Dec 7, 2013 #7

    AlephZero

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    Or in this case, more than 2000 small holes :biggrin:

    (I didn't count them all, but my mental arithmetic is good enough to do 2 x 30 x 30).
     
  9. Dec 7, 2013 #8
    That's awesome, I'll have to keep my eye out for this effect. So it's pretty much a physiological thing, and not an actual optical effect?
     
  10. Dec 7, 2013 #9
    That's a pretty good representation of the effect. There was a more pronounced hexigon visable at the rear of the base mentioned and the holes also at the rear were not nearly as clear.It was more like looking through a lense. I will have to go back and take a picture.
     
  11. Dec 7, 2013 #10
    Yes that seems to fit the picture of what Baluncore was describeing,Moire Fringes seem to be phenomena or a version of it as to what was observed.Definately going to have another look sometime as I am not sure what happens to the view of objects beyond the two sets of holes.
    Or if you can see them.
     
  12. Dec 7, 2013 #11
    Not so sure if it's not an actual optical effect.If we can all see it!
    Would you not call it a psycological rather than pysiological thing if it was not optical.
     
  13. Dec 7, 2013 #12

    OmCheeto

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    :eek:

    How will we know how many holes it takes then?
     
  14. Dec 7, 2013 #13

    Baluncore

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    It is actually a failure to sample at more than twice the highest spatial frequency of the pattern being observed. The result is aliasing.

    When you view one array through another identical array, the divergent rays of light through the near screen samples the far screen at a rate slightly below the angular spatial frequency of the far screen. By standing in the right place you can arrange to see/sample only one cycle of the pattern.

    The idea that it is a magnification is psychological. The magnification is illusory.

    If the screens were independently random it would not work.

    If the holes in two identical screens were of constant area and in identical positions, but all of random shapes, you would not notice any difference to round or square holes.

    If the screens were random but identical it would work but only when viewed from far away for a beat frequency much less than one.
     
  15. Dec 7, 2013 #14

    AlephZero

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    ##N_h##, where ##N_h > 0##.

    The nice thing about being an engineer with a math degree is, you get to choose whether to give practical or theoretical answers :biggrin:
     
  16. Dec 8, 2013 #15
    The idea that the magnification is psychological is a surprise.Being of the opinion that if it looks like a duck quacks a flies like one then what is it.
    I would be gratefull if you could elaborate as it's obviouse the genrall consensus is that it is illusory.If I were to photograph the magnification effect would it show or would that also be an illusion or is it that I am just not grasping the concept.
     
  17. Dec 8, 2013 #16

    Baluncore

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    Which hole is being magnified?
    What is the resolution of the hypothetical lens?
     
  18. Dec 8, 2013 #17

    Baluncore

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    The holes have very sharp sides but the “magnified” image has only the most gentle variation.
    You see much less resolution, magnification should give increased resolution.

    The front screen is a spatial filter. The rear screen is a spatial image.
    What you are seeing is the convolution of those spatial patterns.
    All fine detail has been lost. You can see only the lowest spatial frequencies.
    The low frequencies come from hole area and position, not from hole shape.
     
  19. Dec 8, 2013 #18
    And here is the one dimensional version of the effect that you can hear.
     
  20. Dec 8, 2013 #19
    Magnification does not give increased resolution the opposite is true.I will have to go back and photograph the image and try to post thanks for the imput I hope you can give more info when I manage to post some pictures.Thanks.
     
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