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Light: Wave or Particle? - Reflection

  1. Apr 5, 2006 #1
    I know this has been discussed to death and I acknowledge the fact that there truly is no conclusive answer. However, I have to do my best to outline that light is indeed a particle through its properties of reflection. So far, I've simply come up with the fact that laws of reflection follows laws of motion. Any help regarding other sources of light behaving as a partcle through reflection is appreciated. Also, what properties or experiments decidely supports the corpuscular theory (I know only about Photoelectric Effect). Furthermore, how does light behave as a wave through the following: Propagation, Refraction, Partial Reflection/Partial Refraction, Diffraction and Dispersion.

  2. jcsd
  3. Apr 6, 2006 #2
    Read Richard Feynman's book "QED: the strange theory of light and matter" and you'll be surprised how weird the behaviour of light (and nature) actually is. It isn't a wave nor it is a solid particle; it is something very different.
  4. Apr 6, 2006 #3


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    I hesitate to tackle this because I see a lengthy discussion. The problem here is that there are two participants in such a process - the "photon" and the "reflective medium". To be able to formulate a photon description of the reflectioin process, you have to understand both.

    For example, if you read our FAQ in this section, you will have seen an article regarding the transmission of light through a solid medium from the point of view of photon transport. You will have noticed that not only do you need to know about the behavior of photons, but you also need to know about the properties of the material, i.e. what are phonons and phonons density of states. You can't just use one and ignore the other participant in the picture.

    The same can be said here. For the visible light range, you will notice that the most efficient reflectors are metals. The free electrons in the metals are the ones predominantly involved in reflecting these photons. And to be able to account for that, one needs to understand the electronic band structure of metals, what solid state physicists call the dispersion curve (E versus k graph) of metals. And then, you have to understand the reduced or extended zone scheme to show why only certain k vectors or momentum are allowed in such a transition when a photon is absorbed in the process before it can be re-emitted as a reflected photon with the same in-plane wave vector.

    Now, already this is getting too involved to explain because there's several background knowledge that you have to know to be able to decipher what I have just said. I'm not sure you want that.

    Also take note that a reflection phenomenon is not one of those observations that are used to argue for the validity of the photon picture. So if you are using it as such, then it doesn't present a compelling evidence because a simple wave explanation can do equally well.

  5. Apr 9, 2006 #4
    I'm actually doing a particle-wave debate on the nature of light and was designated to represent the particle team and focus on reflection. I don't need to go in depth necessarily considering the time restraint, just need some fundamental points on my subject. Do you know of any quick demonstrations that may help explain light as a particle under reflection? Also, what things in the wave theory are disprovable in reflection?
  6. Apr 9, 2006 #5
    Light is indeed a particle, and we are all waves!

    Well, according to DeBroglie, people emit about 0.5 Angstroms when walking
  7. Apr 10, 2006 #6


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    In other words " I have this theory that completely contradicts normal physics- please tell me how to prove it?"!!

    Light acts like a particle in reflecting from something LARGE compared with its wave length. There's nothing surprizing about that- ocean waves reflect from very large objects in exactly the way particles do. But they refract around small objects exactly the way light refracts around small objects.
  8. Apr 10, 2006 #7


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    Who exactly is forcing you into such a debate? Whoever this person is, tell him/her that this is the silliest thing he/she can think of.

    There is NO SUCH dichotomy in quantum mechanics. If this person doesn't believe you, ask him/her to open an QM text and point to you where there are two different description of light in QM. There isn't! Light has one, SINGLE consistent theory in QM. That's it! We do not have to switch gears to explain the photoelectric effect, and then switch gears again to explain the diffraction phenomenon.

    It is ONLY when we try to explain its behavior using CLASSICAL analogy that we are force to use this "wave picture" and then "particle picture". This is because in classical physics, we simply cannot reconcile these two different phenomena. We want to explain something square using our knowledge of round holes. Thus, we are force to make compromises. We have no such difficulties in QM.

    So maybe you could end your debate (if you are forced to take a stand) by indicating that both QM and Richard Fenyman would answer "Neither" to the question "Is light a wave or a particle?"


    P.S. You may also want to read our FAQ in this section of PF.
  9. Apr 11, 2006 #8


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    Only 0.5A? I think it's more like 10^-20A
  10. Apr 11, 2006 #9
    ZapperZ, it's actually my grade 12 physics teacher and I think he acknowledges the fact that the nature of light is something of its own. The debate either is required in the cirriculum or simply a evaluation strategy.

    Ok, so I think I have reflection covered, I desperately need help explaining Partial Reflection/Partial Refraction now using the particle theory. I have come to realize that there is not much, but saying that will surely not help my mark. Firstly, can someone explain to me with clarity what this phenomenon encompasses. What did Newton mean when he said light sometimes arrives in a "fit" at the surface of reflection and refraction. What other things in the particle theory help explain this subject?
  11. Apr 11, 2006 #10

    yeah about 0.5Angs if the person is walking an average of 3 miles/h and weighs about 60-70 kg.
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