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In my physics notes there is a section on line spectra, and describing how absorbance line spectra can by used by astronomers to find out what gases are present in a star, should it be emitting white light from its core.

I understand the concept of the absorbance by the atoms of photons of only specific energies, associated with the different energy levels of the electrons within it. In that sense, I can see how the absorbance line spectrum would form.

The notes then speak about how the excited electrons return to the ground state and release a photon.

My question is this: surely that light would “counteract” the absorption? If the absorbed photons are just re-emitted at a later stage so wouldn’t that just “fill in” the spectrum again?

While editing this question I read my notes again and checked the wording closely. It doesn’t specify that the emitted photon has to be of equal energy as the absorbed one - is that the answer? Can an electron drop down a few energy levels (emitting a photon) without losing all of its energy?

Many thanks, Hugo

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    $\begingroup$ Please note that it is not electrons that "absorb and emit photons", but the atom as whole quantum entity. Free electrons scatter off photons and can emit photons when accelerated. The answer by Dale here is relevant to the question physics.stackexchange.com/questions/667880/… $\endgroup$
    – anna v
    Sep 29, 2021 at 7:59
  • $\begingroup$ @annav Thank you for your comment and for the link to that answer, I will edit the question. $\endgroup$
    – UnrulyTank
    Sep 29, 2021 at 8:02

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Yes, materials can absorb high energy photons and release it as multiple lower energy photons.

But also important is that the direction of the release is not the same as the absorption. If we have a beam of particular power on an absorptive material that can come to equilibrium, it must radiate that same power. But if the emission is spherically symmetric, the power in any one direction will be much lower than that of the initial beam.

It may only be compared the background of the brighter source that the material appears to be an absorptive spectrum. If viewed from another angle with a dim background it may show an emission spectrum.

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