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Spectral Lines - How and Why?

  1. Jul 2, 2008 #1
    I don't really understand how we observe spectral lines of a particular element and why they are there? Can anyone help?

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  3. Jul 2, 2008 #2


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    The spectral lines appear because electrons in atoms are only allowed to occupy certain energy levels that are characteristic of the given element. i.e. Electrons in a given element can only have certain energies. When an atoms electrons are excited to higher energies they eventually fall back down to lower energy levels, giving off their energy as photons (light).

    Now, the frequency of a photon given off by one of these electrons is determined by the following:


    where E is the energy of the photon and f is it's frequency.

    Since the electrons are only able to give of specific amounts of energy (the energy differences between two allowed energy levels) the electrons will thus only be able to give off specific frequencies of light, those corresponding to transitions between allowed energy levels. These specific energies/frequencies are different for different elements and are the cause of different spectral lines for different atoms(where each line corresponds to a different frequency or wavelength).

    Does this explanation make things any clearer?
  4. Jul 2, 2008 #3
    Understood, thanks a lot, that was a most helpful explanation.
  5. Jul 3, 2008 #4
    If the emitted photon frequency was specific, i.e. exact, then the position of the photon would be anywhere in the Universe from Heisenberg's Uncertaintity Principle. (delta p * delta x = a constant).
    Is there a slight spread in the frequency of the emitted photon in practice?
  6. Jul 3, 2008 #5


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    There is actually a narrow spread or distribution of frequencies for each atomic spectral line. The width of the line is referred to as the natural linewidth, so that the frequency (and hence energy) of emitted photons is not precisely known.
  7. Jul 3, 2008 #6


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    Note, however, that this is NOT due to QM effects.
    The linewidth of a spectral line is the inverse of the lifetime of the state; this is a simply a consequence of basic Fourier analysis and the relation between time and frequency: If something is localized in time it has to be delocalized in frequency and vice versa (i.e. short lifetimes give broad frequency distributions); this is sometimes known as the "mathematical uncertainty principle" and has nothing as such to do with physics (it is also important when e.g. designing filters).
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