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Experiment showing energy levels of light are quantized

  1. Dec 10, 2015 #1
    Although I have not been thinking very much about actual physics for while now I suddenly got the urge to read a little bit about it and I decided to start by checking out the birth of quantum mechanics again.

    In the book quantum theory by Bohm in chapter 1 the ultra-violet catastrophe and its resolution are explained. It is also mentioned there that Planck originial idea was that the material oscillators in the wall of the cavity only emit energy at certain discrete levels. In the book as well as in my recollection of verious quantum mechanics courses I recall one usually also assumes that the light itsself can only receive energy in quanta. In the book Bohm reasons as follows. He says that, since the only light we can detect couples with matter and the material that the walls of the cavity are made of does not matter, it is equivalent to assume that the light only absorbs energy in quanta or to assume the material only emits energy in quanta.

    He then continues to demonstrate the quantization of matter with an example about the dependence of the specific heat of a crystal on temperature. I thought this reasoning is a little bit strange since it seems Occams razor would suggest only deciding that matter must emit quanta. It actually seems like Bohm's reasoning tells me no experiment could show that light always absorbs energy in specified quanta. In which case it confuses me that I seem to have always been taught this.

    So I guess my question is if someone understands my confusion and can help me, or more precisely. Is there some other experiment that shows that the allowed energies of specifically light (of some frequency) only take a discrete set of values?
  2. jcsd
  3. Dec 10, 2015 #2


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    I think that you have mixed things up here, because you are combining two different phenomena into one.

    First, there is the quanta of light. This is where the energy carried by light come in clumps. Initially, the photoelectric effect is a strong evidence to show this. Nowadays, the which-way experiment, and the photon antibunching experiment are very strong evidence for the existence of photons.

    The other question is the discrete energy levels. This is not due to photons but rather the discrete energy level of the system in question, such as an atom. This was known even before the idea of photon or quantum mechanics came into existence. The spectrum of light emitted or absorbed by these atoms are well-known and is the clearest evidence for it. How do you think we identify atoms and molecules from a substance noawdays?

  4. Dec 10, 2015 #3


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    This is not true in general. It is true in a cavity, and every laser, microwave, ... resonator demonstrates this nicely.
  5. Dec 11, 2015 #4
    Thank you both (ZapperZ and mfb) for the responses. However, I am still a bit confused.

    This I had indeed considered and it is very possible that my terminology is messed up. In the end my question is really more about the allowed energies for the radiation, not about the particle nature of photons (which it seems to me these experiments are about, but I may be confused about that). I hope you understand my question below and can possibly elaborate about how these experiments might answer that question.

    I think this is rather what my question is about. In the book Bohm analyzes the ultraviolet catastrophe and concludes that we may treat the radiation in the cavity as a collection of harmonic oscillators. He then seems to imply that these oscillators are only allowed discrete energy levels. Apparently Planck's original hypotheses is about the energy levels in the walls of the cavity (so the atoms), which is also what you seem to say here.

    The question I have is caused by the fact that Bohm also mentions that since all the radiation coming from matter with discrete energy levels would have discrete energy levels it does not matter whether we assume the harmonic oscillators making up the radiation have these discrete energy levels or the material (since the material of the walls does not matter). Somehow I also had the impression that it is then implied that all radiation
    has this property of discrete energy levels.

    So these experiments show that it is true in a cavity? Then what experiments show that it is not true in general?
  6. Dec 11, 2015 #5


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    You can never verify that there is an infinite number of possible energy levels, as you cannot count to infinity. There is absolutely no indication of any steps, however. There are also strong theoretical arguments against it. For example, imagine photons energies would occur in steps only for some observer A. Take observer B moving slowly relative to A: he would see different energy levels, and to make it worse the energy levels woud depend on the direction. The physics would prefer one reference frame - again something never observed anywhere.
  7. Dec 11, 2015 #6
    I am a bit confused about this, my question was not about whether or not there was an infinite amount of energy levels, only if there is a discrete
    set of levels. Can I interpret this as saying that this is also unverifiable or only that it has not ever been verified.

    I don't get this. Why would the photon suddenly have no steps if observer B makes the observation?
  8. Dec 11, 2015 #7


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    That is the same question. Discrete levels are finite (at least if you restrict the range), a continuum does not have a finite number.
    It is impossible to rule out steps of arbitrarily small size.
    It would have steps, but different ones, and the step positions would depend on the direction. It is expected that physics is the same for all observers, and thousands of measurements tested this with extremely high precision.
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