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A simple question

  1. Aug 30, 2004 #1
    I have asked this question in this forum twice before and received no meaningful answer. So I thought I would give it one last try and perhaps explain the question in a little more detail to explain what information I am looking for. I honestly did not think I would have to use any significant detail in a forum of physicists, but then I thought that maybe people in the forum don’t really understand what I’m getting at: hence, more detail in the question. If I receive another completely inadequate answer I’ll have to conclude that all the physicists who read this forum do not understand what they know and move on to another physics forum, maybe a more international one. So, here’s the question again:

    Regarding the emission and absorption frequencies of the hydrogen atom. I’ve read that the emission and absorption frequencies are the same for a jump (up or down) from the same quantum levels. While the emissive frequencies are obvious, the absorption frequencies are not.

    My question is; how is it known that the absorption frequencies are the same as the emissive frequencies? Given the experiments that are done to stimulate the hydrogen atoms to make these quantum jumps, I would assume that when the atoms absorb photons, they are absorbing them from a source. What is the source? If the source is ambient light then this could be easily verified by simply conducting the experiment in a dark room with absolutely no light.

    If the experiment continues to emit light then the conclusions regarding absorption are incorrect and there are a couple of possible explanations. The first would be that the applied voltage used to stimulate the hydrogen atoms are part of the equation. In other words the voltage is causing the up jump. Another possibility is that the experiment is ‘feeding on itself’. That is, it is taking something from the surrounding hydrogen atoms in order to make the jump. This could be verified by weighing the test sample before and after to see if anything is lost. (This should have been done anyway.) If it is feeding on itself then it would eventually wind down.

    If absorption does take place from ambient light then, in a completely dark room, there should be no emissions at all. (Don’t stumble over the furniture.) Once verified that the absorption process needs ambient light, the frequencies could be verified by emitting monochromatic frequencies (in the dark room) at the emissive frequencies to sustain (or start) the emissions. If the conventional emissive frequencies (410nm, 434nm, 486nm, or 656nm (or the newer infrared)) do not start the ball rolling then other monochromatic frequencies can be used until the actual absorption frequencies are discovered. Since Balmer's math was written to explain the experiments, it is not a reliable source of evidence, as well Lyman's or Paschen's or Bohr's math.

    You can email me at doug@m16eagle.com if you don’t want to use this forum, especially if you think that I have been so stupid that you prefer not to embarrass me publicly. I am good at stupid, sometimes.
  2. jcsd
  3. Aug 31, 2004 #2
    I’m not a physicist but have been reading it with interest. I must admit I don’t quite understand the detail of your question but I think the main question is “how is it known that the absorption frequencies are the same as the emissive frequencies?”

    As far as I understand, the absorption frequencies and the emissive frequencies of hydrogen (or anything else) are measured separately. The absorption frequencies are measured in this way: A beam of polychromatic light of continuous frequencies in the interested range is directed to pass through the test sample (e.g. hydrogen gas) and then into the spectrometer (e.g. a prism). If there is an absorption, a dark line corresponding to the absorption frequency will appear in the previously continuous spectrum. This frequency can be measured and checked whether it’s the same as one of the emissive frequencies of the tested sample.

    If a monochromatic light of known frequency is used instead, its intensity will diminish greatly after passing through the test sample if the frequency is the same as one of the absorption frequencies of the test sample, but will not change if it is not. The absorption frequency can thus also be determined in this way.
  4. Aug 31, 2004 #3
    Emission lines are acquired in laboratoria by using an excited sample of certain atoms (e.g. hydrogen). The emitted light coming from the de-excitation-process is then "printed out" on some screen that is sensitive to incident photons. One can use a photon-multiplyer in order to convert the incident fotons into electric current. Convert is perhaps not well chosen, yet i think you get the point.

    Absorption lines are acquired from continuous light (this is light of all frequencies) passing through some sample of atoms. The dark lines appearing in the spectrum of the light correspond to those frequencies that have been absorbed as the light was passing through the sample.

    Comparing the two spectra can give much info on the absorption and emission lines.

  5. Aug 31, 2004 #4

    In order to get some kind of excited sample of atoms, one can do the following : use light or particles like electrons, and let them collide onto the sample. The energy associated with the incident particle is then used to excite the sample. I mean that an electron from one energy-level takes the energy and uses it to go to some higher energy-level, and leaving an "opening" on the level it came from. This is the excited state. De-excitation occurs when this opening is filled up by some other electron from an higher energy-level. The energy loss of this electron gives rise to the emitted radiation , which is the emission line.

    QM tells us that is energy-levels of electrons in an atom are discrete. This means that if you want an electron to go from one level to another, it has to acquire a specific amount of energy to do so. Just this specific amount of energy will do, nothing more, nothing less.

    The width of an energy-level is the same if you go from level 1 to level 2 (absorption, the electron takes the energy of the incident particle in order to go to level 2), or if you go from level 2 to level 1 (emission of radiation)

    To be complete I must mention that the energy of the incident particle must exactly correspond to the energy-difference between level 1 and 2 of the electrons in the atom. If this condition is obeyed then excitation will occur, otherwise it will not.

  6. Aug 31, 2004 #5


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    Just a thought, but hasn't this already been done? It looks as if what you'rre talking about here is a good discription of a LASER. Monochromatic light is used to excite atoms of a particular element, which absorb light of that frequency and then emit it again, still at that same frequency. That would seem to give confirmation that the element in question absorbs ligth at the predicted frequency, wouldn't it?
    Last edited: Aug 31, 2004
  7. Aug 31, 2004 #6
    I agree with the remark of Lurch...

  8. Aug 31, 2004 #7


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    There is a property of atoms emitting light called "persistency". A time delay between absorption and emission. e.g. glow in the dark toys


  9. Sep 4, 2004 #8
    Well, Thank you. All of you. After I read your posts and brushed out the cobwebs from my brain I remembered reading about the dark lines many, many years ago when I was studying other phenomena. I recently searched all over the internet trying to find just this information, but none of the sites mentioned using polychromatic light and searching for dark lines.

    I still wonder if the experiment were done in a dark room (using high voltage as the catalyst), would the emissions start at all since absorption could not take place without a light source. Has this ever been tried? If so, please let me know. I assume that a voltage would be the best approach since a heat source would be a source of photons. However, I am also aware that the voltage could supply the energy needed for a quantum jump. Maybe a better question would be: Has anyone performed an absorption test in such a way that proves that a quantum jump (up) cannot take place without a source of photons? If this has been done, please let me know. Thank you all.

    I have another related question that I have not been able to track down that I will list as a new thread with the title 'another question'.
    Last edited: Sep 4, 2004
  10. Sep 4, 2004 #9


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    I do not understand the question. The way I read it, it sounds silly, so you must mean something else. I read it as: Has anyone experimentally shown that light cannot be absorbed if there is no light ?, which is, eh, bizarre.
    However, if your question is: can there only be an excitation with light ? Then the answer is a clear NO. Fluorescent lamps are an example: an electric current through a diluted gas excites the atoms, which then emit photons.

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