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Hydrogen spectra

  1. Jun 27, 2015 #1
    Absorbtion spectra can be be observed by passing a WHite light ( light containing Wavelength from appx. 400nm to 700nm).

    Now here's My confusion!
    if We pass this Light from Hydrogen, It should not show dark lines in continous band. ..... (as to obsorb the Light of Visible region, It's electron first need to reach the second orbit. Only then it can obsorb the frequencies of Visible region.)
    now to reach second orbit, It needs wavelength corresponding to UV region, but white light doesnt have it.

    No article or Textbooks mention any such case! Can someone please help me clearing this doubt.
  2. jcsd
  3. Jun 27, 2015 #2


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    I couldn't say for sure, but I have one good guess.

    The transition to the first excited energy level of hydrogen is about 10eV
    Boltzmann's constant times room temperature (in Kelvins) is about 1/40 eV

    Using Boltzmann statistics, at room temperature, the fraction of hydrogen atoms that just happen to be in the first excited state is:
    [itex]e^{-10/(1/40)}=e^{-400}\approx 1/10^{173}[/itex]
    which means it's exceedingly unlikely that even one atom in a gas cell at room temperature will absorb the white light.

    However, if that vapor cell were white hot (say 58,000K), that fraction is only
    [itex]e^{-10/(20)}=e^{-2}\approx 13/100[/itex]
    And a significant, fraction of hydrogen atoms will absorb and re-emit white light.

    The reason that we can see the spectra at temperatures much cooler, say like the surface of the Sun (5,800K) is that there are other mechanisms that excite atoms, such as collisions with other atoms, and bombardment with stray electrons.

    For example, the colored bands observed from a hydrogen lamp are a product of a huge electric field ripping electrons clean off hydrogen atoms, and the energy released when electron and proton recombine to make a hydrogen atom again.
  4. Jun 28, 2015 #3
    So it means, Hydrogen shouldn't show absorption Spectra At normal temperature, But havent heard of any such case.

    They all say ... dark lines will "ALWAYS" appear corresponding to The lines in line spectra.
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