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B Diffraction of light, the mechanism

  1. Dec 1, 2015 #1
    Hi everybody,

    At wiki, I was astonished to read this about diffraction of light :

    "In Quantum Mechanics, it is a remarkable fact that the mechanism of diffraction is based on radiating particles of a medium, while quantum mechanical waves have no transmitting medium. As being part of the Wave-particle duality, this has no physical explanation."

    Does it mean that diffraction of light, a phenomenon that supports the whole quantum theory, has been left unexplained? There is no mechanical explanation for the change in direction of the light rays on the outskirts of a slit?

    Isn't that premature to build such a permanent structure on such an unstable ground? Why give it a mainstream status when it can crash anytime?
     
  2. jcsd
  3. Dec 1, 2015 #2
    The mechanical rules of the universe are discovered through experimentation. The rules discovered for atomic particles do not follow the rules that were outline by Isaac Newton. So new rules were written to describe what actually happens when these experiments are conducted. These new rules are called quantum mechanics.
     
  4. Dec 1, 2015 #3

    ZapperZ

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    What is premature is to accept the words from a flawed source as Wikipedia and think that it is the Gospel.

    Have you read, for example, the Marcella paper that derived this for 1 slit all the way to multiple slits?

    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    Did that Wikipedia author/s even refered to the existence of such a paper that clearly contradicted to what you quoted?

    Zz.
     
  5. Dec 1, 2015 #4
    Remember the Babinet principle.
     
  6. Dec 1, 2015 #5

    bhobba

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    Just as a follow on from Zappers excellent reply and link, when reading something from a source like Wikipedia that leaves you scratching your head best to come here for the answer.

    There are some areas of QM (and parts of other areas of physics and math as well) that even highly reliable sources such as well respected textbooks get wrong because the complete answer is beyond the level they are aimed at. This will sometimes lead a good thinking student astray.

    Thanks
    Bill
     
  7. Dec 2, 2015 #6

    Nugatory

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    The mechanism behind the diffraction of light has been known for centuries. It's a routine third-semester topic in an undergraduate physics program.
     
  8. Dec 2, 2015 #7
    Hi Scott,
    Yes, the new rules work fine, but they are maths rules, not mechanical ones. Light is considered to go straight line in vacuum, and the wave analysis uses points along a slit to show the different distances traveled by the different rays, points that are considered as source points, which means that from those, light starts traveling in all directions without the information that it is crossing a slit, thus without knowing that it is. I did not know that before I read this article on wiki. I find it interesting, because it means that the real reason for the change in direction remains to be found, which could lead to an unknown property of light or property of matter.

    Thank's for the link ZZ, its maths are interesting, but it doesn't give any physical mechanism for the change in direction either.

    Hi PK,
    With an opaque body, the same question remains: what is changing the direction of the rays if it has no physical interaction with the body?

    Hi Bhobba,
    This discovery did not trouble me, on the contrary, I found it exciting to learn that such a fundamental phenomenon was still physically unexplained.

    Hi Nugatory,
    On the net, I did not find any other mechanism than the analogy with the physical waves mechanism, and usually the author adds that its only an analogy. Here is wiki again on that mechanism:

    "Diffraction arises because of the way in which waves propagate; this is described by the Huygens–Fresnel principle and the principle of superposition of waves. The propagation of a wave can be visualized by considering every particle of the transmitted medium on a wavefront as a point source for a secondary spherical wave. The wave displacement at any subsequent point is the sum of these secondary waves."
     
  9. Dec 2, 2015 #8

    ZapperZ

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    Physical mechanism? The HUP isn't enough of a physical mechanism? And what's so "interesting" about the math? It is something any undergraduate would have seen before.

    And besides, what is the "physical mechanism" for diffraction using the classical wave treatment? You'd consider something like this as a "physical mechanism"?

    This is no different than the Marcella treatment of the single slit! Yet, you'd accept this but not that?

    Zz.
     
  10. Dec 2, 2015 #9
    There is nothing particularly quantum about diffraction. It is fully explained even with the classical theory of EM fields.
     
  11. Dec 2, 2015 #10

    ZapperZ

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    It is "particularly quantum" if the source emits, on average, single photons, or if it is a stream of quantum particles such as electrons.

    Zz.
     
  12. Dec 2, 2015 #11
    Yes, but then you assume something "particularly quantum" to start with, so everything becomes "particularly quantum"...:wink: What I meant was that diffraction can be explained classically, thus contradicting the OP's claim that it is "a phenomenon that supports the whole quantum theory,".
     
  13. Dec 2, 2015 #12

    Nugatory

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    Either the article you're reading is just plain wrong (there's a reason why wikipedia is not an acceptable source under the PhysicsForums rules) or you're misunderstanding it. You are describing some of the many empirically discovered rules, such as the Huygens-Fresnel principle, that were used to predict the behavior of light before the mechanism behind that behavior was known. However, we now know that all of these rules follow from the laws of classical electromagnetism: if E&M behaves according to those laws light will travel in a straight line in a vacuum, it will diffract when it passes through a slit, it will diffract when it crosses the boundary from one medium to another.

    What's not found? Reflection is another phenomena that is predicted and explained by the laws of electricity and magnetism.
     
  14. Dec 2, 2015 #13
    By physical mechanism, I meant one that, for example, explains why the surface waves on water change directions when passing by a solid body. There is a link between sideways molecules of water that is not found between sideways photons. If the extremity of a water wave gets in the air while passing by a body, its easy to understand that its weight will create a new source of waves while falling sideways to the direction of the main wave. But there is no weight or pressure to force the photons from a wave front to begin traveling sideways. After a slit, photons should continue straight line, but they don't. If we take for granted that nothing links them transversally to the wave front, then we have to consider that each photon is a wave source, which means that we should see a laser beam passing by in vacuum, which is unfortunately not the case.

    To me, reflection and refraction are more easily understandable since they depend on light being absorbed and reemitted by the atoms, which are real sources of light that we can observe from all directions.
     
  15. Dec 2, 2015 #14
    It seems you have a mental picture of photons as classical particles. They are nothing of the sort.
     
  16. Dec 2, 2015 #15
    I thought the OP was rated "A", but I am far from being rated that high. I like physics, and I like thinking, that's about it! :smile:
     
  17. Dec 2, 2015 #16
    Whether photons are classical particles or not doesn't change the data though, and the data shows that laser beams do not radiate transversely to their direction in vacuum, so to me, it seems that the explanation that uses each photon as a wave source in the slit experiment is not corroborated by the data, unless what is meant is that interference between sideways sources causes a blackout transversely to the beam. Is that what is meant?
     
  18. Dec 2, 2015 #17
    Photons are excitations of the (quantized) EM-field, so it is easiest to not think of them as particles at all. The idea of regarding each point in a field as an independent field source is simply a mathematical convenience that in no way implies that a photon is a "source" that should radiate light. Everything about diffraction is explained by the dynamics of the field.
     
  19. Dec 2, 2015 #18

    Nugatory

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    Well, you're the OP so you're the one who set the level. I'll reset it to the B level appropriate for discussions that use less math than you'd encounter as an undergraduate physics student.
     
  20. Dec 2, 2015 #19

    ZapperZ

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    Of all my comments and questions at you in that post, this is the ONLY thing that caught your eye?

    Again, what's different between the Marcella description of the single slit versus the classical wave description of the single slit? You appear to accept the latter as having a "mechanism", but you don't accept the former and just thought that the math was "interesting".

    Do you even know what a "mechanism" means?

    Zz.
     
  21. Dec 2, 2015 #20

    Nugatory

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    Photons are not what you think they are. They don't travel in the sense that you're thinking about, they don't change direction, diffraction doesn't happen because they start to move sideways, and above all else you must not imagine that a beam of light is a stream of photons passing by the way a river is a stream of water molecules passing by.

    Unfortunately, it's a lot easier to say what a photon is not than to say what a photon is - to explain it properly you need quantum electrodynamics, which is more than you'll find in a four-year undergraduate physics program. The best layman-friendly explanation that I know of is this book by Richard Feynman; it's one of the very few exceptions to the general rule that everything you read about light and photons that didn't come from a quantum electrodynamics textbook is wrong (and despite my glowing recommendation, feynman's book is still a popularization and no substitute for the real thing).
    As I said above... photons aren't what you think they are. The mechanism of diffraction has absolutely nothing to do with photons being a wave source, and indeed they are no such thing. It's worth noting that the mechanism of diffraction, which is based on classical electromagnetism, was well understood many decades before the discovery of the photon.

    As this entire thread is based on a misunderstanding of what the science actually says, and this misunderstanding has been pointed out, we can close the thread.
     
    Last edited: Dec 2, 2015
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