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    An emission spectrum is the result of electrons being excited or 'boosted' from one energy level to another. As they fall back down to their original energy level, they release a photon of light with a frequency corresponding to the energy difference. This releases certain colours of light which is what you see through a spectroscope.

    An absorption spectrum occurs when a light source (e.g. a star) shines through the sample (say, a planet's atmosphere) and the atoms of the sample absorb photons with frequencies corresponding to their possibly energy level differences. This accounts for the black lines you see through a spectroscope. This absorbed energy excites electrons to a higher energy level corresponding to the frequency of the photon.

    So far, so good.

    What I don't get is why the electrons don't return to their ground state after being excited, releasing a photon with the same frequency as that which was absorbed - in other words, surely the atoms should absorb and emit exactly the same colours of light, and surely there should be no black lines seen through a spectroscope?

    My chemistry teacher couldn't answer this, I hope someone here can.
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    (Original post by jmenkus)
    An emission spectrum is the result of electrons being excited or 'boosted' from one energy level to another. As they fall back down to their original energy level, they release a photon of light with a frequency corresponding to the energy difference. This releases certain colours of light which is what you see through a spectroscope.

    An absorption spectrum occurs when a light source (e.g. a star) shines through the sample (say, a planet's atmosphere) and the atoms of the sample absorb photons with frequencies corresponding to their possibly energy level differences. This accounts for the black lines you see through a spectroscope. This absorbed energy excites electrons to a higher energy level corresponding to the frequency of the photon.

    So far, so good.

    What I don't get is why the electrons don't return to their ground state after being excited, releasing a photon with the same frequency as that which was absorbed - in other words, surely the atoms should absorb and emit exactly the same colours of light, and surely there should be no black lines seen through a spectroscope?

    My chemistry teacher couldn't answer this, I hope someone here can.
    Two things:

    - the electron does drop down, but not instantaneously
    - it doesn't necessarily have to drop down in one step, it can drop down in a few steps, releasing photons of different frequencies from the absorbed one
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    (Original post by jmenkus)
    An emission spectrum is the result of electrons being excited or 'boosted' from one energy level to another. As they fall back down to their original energy level, they release a photon of light with a frequency corresponding to the energy difference. This releases certain colours of light which is what you see through a spectroscope.

    An absorption spectrum occurs when a light source (e.g. a star) shines through the sample (say, a planet's atmosphere) and the atoms of the sample absorb photons with frequencies corresponding to their possibly energy level differences. This accounts for the black lines you see through a spectroscope. This absorbed energy excites electrons to a higher energy level corresponding to the frequency of the photon.

    So far, so good.

    What I don't get is why the electrons don't return to their ground state after being excited, releasing a photon with the same frequency as that which was absorbed - in other words, surely the atoms should absorb and emit exactly the same colours of light, and surely there should be no black lines seen through a spectroscope?

    My chemistry teacher couldn't answer this, I hope someone here can.
    Adding to what Kyle wrote...

    .. the other factor is that in the case of relaxation directly down to the original level emission could occur in any direction, whereas your absorption is being measured from one only.

    Hence only a tiny fraction of the emitted light would be seen.
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    It can do exactly that, there will be a spectral line that corresponds to that transition, however as there are many other possible transitions, there will be lots of other spectral lines as well. So the excitation might be from 1->4, then the emission from 4->2 then 2->1 with the emission of 2 photons of different wavelength.
 
 
 
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