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I have read that for Rayleigh scattering the photons are absorbed by the atmospheric particles then re-emitted (albeit very quickly) and the shorter wavelengths scatter better which is why the sky is blue. Am I right in saying that the sky is blue because the energy level transitions available in oxygen allow it to absorb and re-emit (scatter) the blue light from the sun? And that the reason why we don't get a violet sky, from the more abundant nitrogen in the atmosphere (nitrogen emission spectrum when mixed appears violet), is that there is simply less violet light in the sun's spectrum?

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Rayleigh scattering is scattering. The photons are not absorbed and the scattering can take place at all wavelengths. The scattering is caused by both oxygen and nitrogen in the atmosphere and they have very similar Rayleigh scattering cross-sections.

The sky is not violet because there is little violet in the solar spectrum, which is what is being scattered, and the eye is not very sensitive to that violet light. See Why is the sky not purple?

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  • $\begingroup$ So what actually is scattering then? there must be some interaction of the incoming light and the atoms they are incident upon in order for the direction of light to be changed? Wikipedia (perhaps not the most reliable or the context is different) says the light is absorbed and re-emitted - see first line of the article: en.wikipedia.org/wiki/Scattering $\endgroup$
    – tomd7824
    Commented Aug 23, 2020 at 17:08
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    $\begingroup$ @tomd7824 Well, that's a case where Wikipedia is not so reliable. It's hard to describe the difference between scattering and re-emission in a short, succinct sentence. There are subtleties that need to be explained. The mathematical and pictorial descriptions of scattering suggest a very tempting interpretation as absorption and re-emission, but I think that most physicists would say that it doesn't happen that way. One distinction is that in scattering the scattered light maintains a phase relationship with the incoming light. In re-emission, there is no phase relationship. $\endgroup$
    – garyp
    Commented Aug 23, 2020 at 17:38
  • $\begingroup$ okay, thanks, that's helpful. is there a reason why all frequencies can be scattered but absorption and re-emission is quantisied? $\endgroup$
    – tomd7824
    Commented Aug 23, 2020 at 18:28
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    $\begingroup$ tomd7824: Maybe you can think of it this way: Any light due to its oscillating $\vec{E}$ starts an oscillation in the atom or anything its interacting with (making it behave like an oscillating dipole) these oscillations, damped by the partial polarized charges present at each dipole end, emit em waves which we call scattered waves . In cases when the incoming frequency matches the difference in discrete levels it causes resonance (resonant frequency) which is reflected as temporary absorption. $\endgroup$
    – Lost
    Commented Aug 24, 2020 at 0:43
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    $\begingroup$ Well, one difference is in the angular distribution of the scattered radiation. Spontaneous emission can be in any direction. Scattering follows the dipolar distribution and is polarised in the same direction as the incoming radiation. @tomd7824 $\endgroup$
    – ProfRob
    Commented Mar 30, 2021 at 18:51

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