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EM radiation question

  1. Apr 4, 2005 #1
    hi guys, having a bit of trouble answering this physics question. Wondering if any of the experts can help me out.

    Can the electromagnetic radiation produced by a hot gas of hydrogen atoms be exactly of one frequency?

    not sure if this is true or not. They say also that the atoms make a transition from state n=2 to n=1 and that all atoms are moving.

    hope someone can help

    oh, a similar question involving state transitions

    A hydrogen atom undergoes a transition from the state n=3 to n=2 and then another transition from n=2 to n=1. Two photons are created due to these processes. Which photon has the higher frequency?

    any thoughts?
    thanks
     
  2. jcsd
  3. Apr 4, 2005 #2

    chroot

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    No, the hot hydrogen gas cannot produce a single frequency. The hydrogen atoms have many energy levels to which electrons are randomly excited by thermal energy. As the electrons decay to lower energy levels, the atom emits photons of different frequencies.

    As for the second question, the distance (in energy) between n=1 and n=2 is larger than the distance between any two higher energy levels.

    - Warren
     
  4. Apr 5, 2005 #3
    how does dat explain y it cannot have a frequency of one thou
     
  5. Apr 5, 2005 #4

    chroot

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    The hydrogen atoms have many energy levels to which electrons are randomly excited by thermal energy. Each of these energy levels (transitions from one to another, actually) will result in a photon with a unique frequency. There's no way to make the electrons go into only one energy level with thermal excitation.

    - Warren
     
  6. Apr 5, 2005 #5

    dextercioby

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    Warren,u could use another photons :uhh: No need to heat the gas...

    Daniel.
     
  7. Apr 5, 2005 #6

    chroot

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    Please read the question before responding.

    Can the electromagnetic radiation produced by a hot gas of hydrogen atoms be exactly of one frequency?

    - Warren
     
  8. Apr 5, 2005 #7

    SpaceTiger

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    In theory, you could produce a gas that within a given interval of time, only emitted lines at one transition, but it would either be an extreme statistical improbability, a tiny interval of time, or an extremely contrived situation (say, a near absolute zero gas illuminated with photons of one energy). The questioner used the term "hot" gas, so I think it's fairly natural to assume that they weren't interested in such situations.
     
  9. Apr 5, 2005 #8

    dextercioby

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    I just gave a counterexample to your statement...There is "a way to make electrons go into only one energy level with thermal excitation".

    I had read the question (and your answer) and i was addressing you...

    Daniel.
     
  10. Apr 5, 2005 #9

    dextercioby

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    I know what the questioner was interested in knowing & i think Warren answered pretty well.He just made an assertion which is not valid.

    Daniel.
     
  11. Apr 5, 2005 #10

    chroot

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    The questioner is not asking about lasers, dexter. Drop it.

    - Warren
     
  12. Apr 5, 2005 #11
    a little confused

    thanks for all the input guys. sorry for posting in two different forums

    i'm just a little confused, since the point was brought up that the gas is hot and in this state there is really no chance that the radiation will be of only one frequency

    but what is this about lasers? you see these are conceptual homework questions not worth much but are very important in the understanding of this first year physics course, and they seem to throw a lot of trick questions our way.. keeping this in mind, is it still safe to say that the there isn't going to be a single frequency at all for the hot hydrogen atom?

    oh and warren also said this
    "The hydrogen atoms have many energy levels to which electrons are randomly excited by thermal energy. Each of these energy levels (transitions from one to another, actually) will result in a photon with a unique frequency. There's no way to make the electrons go into only one energy level with thermal excitation."

    this can help me with my second question since each transition results in a photon with a unique frequency, and you said the most energetic transition is from n=2 to n=1, does this mean that the photon has a higher frequency at this state or a lower one than from n=3 to n=2.

    thanks
     
  13. Apr 5, 2005 #12

    dextercioby

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    Frequency is proportional to the difference between various energy levels...

    Daniel.
     
  14. Apr 6, 2005 #13
    But since the question specifies that the hydrogen atoms are moving from n=2 to n=1, doesn't this mean they must emit a single frequency?

    If the hydrogen atoms are moving, does this affect the frequency emitted?
     
  15. Apr 6, 2005 #14

    chroot

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    Well,

    A real hot gas will not excite JUST the n=2 state. However, if the only state excited were n=2, then the photons would all be the same frequency.

    Of course, the Doppler effect would cause a spread in the frequencies as received by a detector, since the atoms are moving around at relatively high speeds.

    - Warren
     
  16. Apr 6, 2005 #15

    SpaceTiger

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    This has been mentioned several times recently, but it seems worth mentioning this thread again. There are several mechanisms by which lines are broadened, so in practice, things are never emitted all at the exact same frequency, even in a laser.
     
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