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Consider this image and quotations from Basics of Radio Astronomy for the Green Bank Telescope:

When a continuous spectrum is viewed through some cool gas, dark spectral lines (called absorption lines) appear in the continuous spectrum.

Spectral Analysis]

If the gas is viewed at an angle away from the source of the continuous spectrum, a pattern of bright spectral lines (called emission lines) is seen against an other-wise dark background.

As the radiation passes through a gas, certain wavelengths are absorbed. Those same wavelengths appear in emission when the gas is observed at an angle with respect to the radiation source.

Since the gas absorbs photons and then re-emits them isotropically by my logic there is no reason why there should be any dark lines appearing on the absorption spectrum. Explicitly, I don't understand why the diagram doesn't look like this instead:

Spectral Analysis 2

Since, as I understood it, this should be the case:

enter image description here

Since the absorbed photons in the gas are re-emitted why do we see any dark lines?


marked as duplicate by John Rennie, Kyle Kanos, Community Sep 7 '18 at 11:16

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  • 1
    $\begingroup$ The thing is that the "absorbed" part is re-emitted isotropically, so that only a very small portion can be compensated for in the direction where you observe the absorption lines. Your last picture must be corrected by multiplying the emission spectrum by a very small coefficient. This coefficient is so small that in the end one can neglect it and consider the 1st picture as the correct depiction $\endgroup$ – David Sep 7 '18 at 10:42

When a photon is absorbed, it does not reach your eyes anymore, so for that specific wavelength the image gets darker. In the spectrum this wavelength is suppressed. The reemitted photon is sent off in a random direction. It is extremely unlikely that it will reach your eyes and it will therefore not make up for the lost spectral intensity.

  • $\begingroup$ Thank you for your answer, I was looking at other explanations for this and one of them said it is due to the 'relaxation time' of the deexcitation; but I don't see how this makes sense. Is it false to say that the dark lines are because of a time delay due to relaxation time? (and this is assuming that the photon is reemitted in exactly the same direction it was absorbed in). $\endgroup$ – BLAZE Sep 7 '18 at 10:35
  • $\begingroup$ What you need is loss of coherence between absorbed and emitted photon. If it involves an extremely shortlived excited state the loss of coherence may perhaps be incomplete. $\endgroup$ – my2cts Sep 7 '18 at 11:12

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