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I Diffraction experience at home

  1. Nov 15, 2016 #1
    Hi all.
    Just some moments ago I've experimented the diffraction phenomenon at home, in a situation I would have considered impossible. I did this because a guy that I know had experienced that too and I wanted to reproduce the experience: the diffraction from circular aperture.

    As far as I know the aperture must be as the order of magnitude of the wavelength, and the light should be coherent, but I was able to reproduce the interference pattern by simply using a normal incandescence lamp and a hole of about 1 mm of diameter.
    Now, visible light lies between 380 and 780 nm, while the aperture is 1 mm. Even in the worst case is more than one thousand times bigger. Moreover this light is produced by a incoherent source, so I cannot just figure you how this is possible. The circle was either pretty large but very undefined with the screen far from the aperture, or pretty small but more clear with the screen close to it.

    Can somebody please explain me why?
     
  2. jcsd
  3. Nov 15, 2016 #2

    Drakkith

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    Can you described the diffraction pattern you produced?
     
  4. Nov 15, 2016 #3

    Bystander

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  5. Nov 15, 2016 #4

    OmCheeto

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    This sounds like a pinhole camera, and you are just projecting an image of your source bulb.

    ps. I just did the experiment too, and that's what I saw.
    pps. I double checked with my 635-660 nm laser and I saw some "fringing" around the laser point. I suspect that the light is scattering off of the edges of my pinhole, as I used some white card-stock junk mail.
     
  6. Nov 15, 2016 #5

    OCR

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    :thumbup:... for the link. Thanks.
     
  7. Nov 16, 2016 #6
    So...
    I've found out the the light I used was a led not an incandescence light.
    These were the images that made me think it was diffraction: link1, link2 (especially the last one)

    But then I tried with other two led lights, and I noted that it was as @OmCheeto said, the hole was projecting the shape of the luminous bulb, sorry but I didn't know this effect.

    Secondly, is this effect also responsible for the light halo around the bulb shape? Because the image is enlarged with a halo, so cannot this be considered as a form of diffraction?

    Thank you
     
    Last edited: Nov 16, 2016
  8. Nov 16, 2016 #7

    OmCheeto

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    What color was the inside of the box that the led was enclosed in? It looks to me like the image of the back of a white box.
     
  9. Nov 16, 2016 #8

    Ibix

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    It could also be an effect of the finite size of the hole blurring out the image slightly.
     
  10. Nov 16, 2016 #9

    OmCheeto

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    It could be lots of things. , but I don't think it's a "QM" thing. [edit: everyone, please ignore that I said that.]

    I watched a bucket load of instructional videos on the topic, last night: Interference of electromagnetic waves [khanacademy.org]
    I even watched an instructional video, before this thread even started!: Lec 34: Heisenberg's Uncertainty Principle | 8.01 Classical Mechanics, Fall 1999 (Walter Lewin)

    So today, I made some measurements, made a doodle, and decided, that I have a great admiration, for real scientists.

    how.to.do.bad.QM.physics.experiments.png
     
    Last edited: Nov 16, 2016
  11. Nov 16, 2016 #10

    Ibix

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    Agreed. But what I meant was just ray optics - a pinhole camera only produces a perfect image if the pinhole has zero diameter, strictly speaking. In that case only one ray comes from each point on the source and reaches the screen. For a finite size hole you get a slightly blurred image because a small cone of rays starting at each point on the source can make it through the hole. That gives a slight blur to the image, which would give the source a halo.
     
  12. Nov 16, 2016 #11
    moire patterns are more of an interference effect involving several sources, it is unlikely to be a moire effect through a single pinhole
     
  13. Nov 16, 2016 #12
    @OmCheeto to yes it was a white box

    @Ibix the blur make sense. I made the test with the led of my cell: image on screen, led source
    I observed that the smaller the hole, the clearer the image of the bulb shape is.

    What is a QM thing???
     
  14. Nov 16, 2016 #13

    OmCheeto

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    Just ignore I said that.
    I think I mentioned it, as Professor Lewin described the effect you were seeing as a quantum mechanical(QM) effect, at 32 minutes into the video, that I posted earlier.
    I had never heard of such a thing before, so I thought it was interesting.

    I should probably stop sharing interesting things, when they will only confuse things. :redface:
     
  15. Nov 17, 2016 #14
    Why? It's just that I didn't know the acronym QM, becauseI never used it nor read it. I'm interested in such things but I'm not having the time to see the full video, and I thank you for your sharing
     
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