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Question about single slit diffraction

  1. Apr 19, 2005 #1
    This is in response to some posts to an earlier thread with this title.
    In these posts one question seemed to be that the Huygens idea of scattering from points in the space between the slit edges was too mathematical and that scattering from the slit edges might provide a more realistic description of what was happening.
    My thoughts on this are that the Huygens spatial points in the space between the slit edges are really air molecules and that electrons in these molecules are set in motion by the primary light source and that the interference effects observed on a receiving surface are due to the different path distances from these secondary source oscillations of electrons.
    Thus if the slit edges and other parts of the screen between the primary light source and the receiver surface are painted black(which I recall has been done), scattering from these equally physical sources, in analogy to wave producing disturbances in water, could provide and alternative explanation for the observed interference effects.
    This then suggests another question: Has the single slit diffraction experiment been done in a (partial)vacuum? A related question is: Do the laser beams which show up in typical laser demonstrations also show up when done in a (partial) vacuum?
  2. jcsd
  3. Apr 19, 2005 #2
    Hello Rob,

    Welcome to PF. :smile: This is an interesting topic you bring up. I'd like to talk about this because my physics class is studying this right now ironically. Your reasoning seems to make sense to me. I'm not sure if I follow you in the black screen part though. What would performing an experiment, like the one you described, in a partial vacuum show? The way light travels would be altered since light can't travel through a vacuum; it would only be logical to assume that the effects of the partial vacuum would impact the way light travels.
  4. Apr 22, 2005 #3
    What would a single slit diffraction experiment in a vacuum show? I think the experiment might show that there are no diffraction interference effects due to non material light waves eminating from point sources in the slit as Huygens description was interpreted before the MM experiment persuaded most physicists that no such waves could exist. (And before this Einstein had showed that the photoelectric emission of fast moving electrons from aluminum or whatever due to a source of uv light etc occured too soon after the turning on of the source to be due to the wave theory of light)
    But Feynman says that when a wave description works as in single slit and double slit diffraction etc it is mathematically tantamount to a probabilistic photon description(see his QED or vol3 of his Lectures on Physics) even though there are no actual waves of ether in a vacuum. That is all of the light phenomena explained by waves could be explained by probabilistic photons. And so without going into this mathematics, the currently accepted theory of light implies that there would be diffraction effects.
    Thus the experiment would test the currently accepted theory of light.
    I dont know why you say light can't travel through a vacuum eg outer space. Also Michelson or rather his collaborators after his death published the results of their earlier light speed measurement in a vacuum tube in 1935 (see http://encyclopedia.laborlawtalk.com/Albert_Michelson).
    Regarding the painted black screen and edges of the slit, I recall someone saying that they had done this to prevent reflection of light from these locations that might produce the observed diffraction effects on the receiver screen.
  5. Apr 22, 2005 #4
    Although light is a 'little ' effected by air the experiments are all valid in vaccuo
    light does not need any of our normal media . It will diffract purely on a Geometric basis (as per Hygens wave theory ) .
    The difference in QM is that there, a single photon is considered whereas Hygens did not look at single photons , he did not even know of photons .
    All experiments confirm the wave and particulate nature of light ( in the human view ) --- the probable answer is that other dimensions are involved which we cannot sense or see . See theories such as string theory .
    No one has ever measured a photon ( except in energy ) but not dimensionally
    you have to figure that if two telescpes 10 km apart can have interference fringes ( which they do if their light is combined ) that photons must somehow be Km in width ?????????????.
  6. Apr 22, 2005 #5
  7. Apr 22, 2005 #6

    Doc Al

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    Don't get hung up on Huygen's principle. It works, but not for the handwaving arguments usually given:
  8. Apr 25, 2005 #7
    Yes why it works is the question. According to this other standard explantion, em waves or photons from the source produce oscillations of charge in the screen with the slit and if a plug is placed in the slit, secondary oscillations of charge are produced there also. Then the combined effect of these primary and secondary em waves or photons on the back side of the screen facing the receiving wall, these waves etc cancel each other out and so no field emanates from this side of the screen.
    If cancellation was not complete, we could make the screen and plug a little thicker until cancellation was complete.
    Thus the field at the receiver wall due to the field produced by the screen with the slit in it plus the field due to the plug in the slit plus the field produced by the source is, E(screen with slit)+E(plug) + E(source)=0. But this implies that
    -E(plug)=E(screen with slit)+ E(source)
    That is, if you take the plug out and subtract the field at the receiving wall due to the oscillations of charge in this plug you should get the observed pattern of light strips and dark strips.
    This is the same as the Huygens argument without explicitly saying that point sources are produced in the space between the slits.
    The question remains, if you do this demonstration in a vacuum, will the mathematically calculated wave effects which are mathematically equivalent to probablistic photon effects be demonstrated.
    If not, then maybe the observed pattern is due to actual physical point sources, oscillating charges in the air molecules in the slit as well as to oscillations of charge in the primary light source.
  9. Apr 26, 2005 #8
    The Huygens's idea is a mathematical trick to solve the wave equation, nothing more. The first point: if there is two functions which satisfy the wave equation, then their linear combination is a solution too. The second point : by fixing the light intensity and phase on some area we have a unique light field.
    So we can calculate the light field at any point a combination of fictious light sources.
    If we have some media, we need to take into the account the effect of the media, but light itself does not need any media to propagate. Or, let's say, it can propagate in a physical vacuum.
  10. Apr 26, 2005 #9
    Yes the question here rephrased is what is the effect of the air molecule media ? The fact that light does not need any media to propagate in the vacuum of outer space and in Michelson's last light speed measurment etc indicates that the media is not necessary for light propagation in the single slit refraction experiment,
    The mathematics of the wave function as you say imply that the interference effects are solely due to the movement of light as a wave (or a probabalistic photon)
    But this implication of the wave theory of light applied to these interference effects has apparently never been tested directly.
    The possibility exists that just as you dont see a laser beam in a vacuum you might not see the interference pattern in the single slit or double slit refraction in a vacuum.
    That is the effects of light scattering from air molecules as well as the effects of light propagation in a vacuum are well known and the question is what are their contributions to these single slit and double slit diffraction phenomena?
  11. Apr 26, 2005 #10
    The light scattering on air molecules is negligible. The main effect will come from the small decrease of the light speed in the air. Nowever, if air is not uniform, as it is usually, the light will scatter on fluctuations of the air density, so the total intensity will decrease. The picture would be probably more blurry as well.
  12. Apr 26, 2005 #11
    I dont know why you say the scattering from the air molecules is neglible. This is not the case when looking at a laser beam in air. But all of this is just speculation.
    Maybe a test could be done at Abbess:

    whose largest vacuum chamber is 4'by4'by5'.
    The tests that I recall had the wall about ten feet from the few mm slit so that the distance between intensity minima were large enough to see the differences in intensity-which may make sense considering the formula eg see http://www.drchaos.net/drchaos/Whit/Lab_Manual/node34.html

    So the tests in such a vacuum chamber would somehow have to be scaled down to half this scale etc and a photograph taken of the chamber wall where the interference pattern should be. The photograph could then be enlarged. Maybe this is possible.
  13. Apr 26, 2005 #12
    Laser beams don't show up in air unless the the beam is so energetic that it ionizes air.
  14. Apr 27, 2005 #13
    or the air is dusty.
  15. Apr 27, 2005 #14


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    I don't get it. Are you implying that diffraction and interference patterns have NOT been observed in any kind of "vacuum"? How about, let's say, 10^-10 Torr? Would such a level be acceptable?

    If it is, then may I recommend that you visit your neighborhood synchrotron center and ask them how they select the photon energy/wavelength/frequency that they wish to work with. I mean, they don't call these things interferometer, diffractometer, etc. for nothing. And these things ARE in the vacuum beamline, thankyouverymuch!

  16. Apr 27, 2005 #15
    We are obviously not accelerating electrons or protons or such here and we are not talking about electron diffraction here. So I dont think the single slit (light) diffraction experiment in vacuuo has been done at some neighborhood synchroton center. But if you know of such a case or that it has been done somewhere please tell us what you know.
  17. Apr 28, 2005 #16


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    Who says anything about "electron diffracton"?

    When the electron bunches in a synchrotron passes through a series of undulators or wigglers, it generates a SPECTRUM of light depending on the spacing in those insertion devices. This spectrum is then sent to the experimental beamlines that uses the LIGHT (not the electrons!) for varous purposes, but they have to SELECT what frequency they want! So they have to use various things such as an inteferometer, etc. etc... These are nothing more than a glorified diffraction slits that can similarly be found in an undergraduate labs.

    All of these are done in vacuum!

  18. Apr 28, 2005 #17


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    I think this is a good opportunity to get rid of some strange misconception that people may have on what a "synchrotron center" is. A synchrotron center is a LIGHT SOURCE! Various centers are called The Advanced Photon Source, Advanced Light Source, National Synchrotron Light Source, etc. Their sole purpose is to generate light of all differnt kinds that researches use to perform their work! The range of light can be from IR, to UV, to visible, and all the way to very hard X-ray! The range of research work performed at one of these centers range from material science to basic physics to biological to medicine, etc.

    There are many of these centers all over the world, because it has become one of the most important and useful TOOL in doing a huge amount of crucial research. Info on ALL the synchrotron centers throughout the world can be found at


    Most, especially here in the US, have either open days, or can arrange for a tour. Go see one!

  19. Apr 28, 2005 #18
  20. Apr 28, 2005 #19


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    OK, so NOW, you've lost me.

    1. I'm guessing that we have finally straighten you out that synchrotron centers produce LIGHT and not "electrons" that participate in some "diffraction".

    2. You want "ONE diffraction slit a few mm wide" REGARDLESS of the wavelength of light, and expect to see diffraction pattern no matter what?

    3. The thing that I don't understand is this. Can you look up ANY elementary derivation of, let's say, the Fraunhoffer diffraction pattern of a single slit, and point to me where in that derivation is the factor in which it makes a difference if it was done in air or in vacuum. In fact, I will put it to you that ALL of the physics involved was done by assuming a dispersonless medium, which is a vacuum! I will also put it to you that if you check any modern optics text, the diffraction pattern is actually a fourier transform of the SLIT GEOMETRY. It means that doing this in air or vacuum is irrelevant! I could do this in glass and all I will alter is the separation between the patterns!

    4. This is something ANYONE can try (in fact, shouldn't one have already done this in intro physics labs?). Look at the diffraction pattern from a single slit. Then, using the SAME slit width, do this for 2 slit interference. Compare the two patterns. You will see more bright-dark-bright-dark-etc pattern, but if you pay attention, this pattern is MODULATED by the pattern made by the SINGLE slit pattern! I could do this for 3, 4, 5, gazillion slits. If I pay attention to the modulation, I can easily deduce the pattern made by just ONE slit without having to DO the 1-slit diffraction experiment. Having worked at the NSLS for 3 years doing photoemission spectroscopy, I had to adjust the interferrometer all the time when I change the incoming photon energy from the ring. And I'll be damned if the modulation isn't there. Now unless you are claiming some new physics here, I will put it to you that I don't have to do any single-slit experiment to know what a single-slit diffraction pattern would look like simply based on such modulation.

    5. 10 foot vacuum pipe? Do you have any idea how BIG each one of these synchrotron centers are? The APS is a ring that is 3/4 of a mile in circumference! Each of the beamline pipe from the ring is 10 to 20 meters long! 10-foot vacuum pipe is child's play!

  21. Apr 28, 2005 #20
    Yes and no. One diffractions slit but preferably with visible light and a few mm wide single diffraction slit.

    No. All texts that I have seen say there should be no difference because the cause of the diffraction pattern is the difference in distances from point sources on wave fronts in the ether between the edges of the slit( and that there is a mathematical equivalence between the ether wave description of light and the quantum electrodynamic probabalisitic photon description)
    But a number of people like myself have wondered if the cause is due in part or wholly to oscillations of charge in air molecules in the slit as well as in the primary light source. A vacuum version of the experiment would clarify this.
    Last edited by a moderator: Apr 28, 2005
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