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I am taking an intro to astronomy class, and have touched upon absorption and emission lines and etc, the prof asked this question in class and got me thinking.

I would want to say no, because one photon should stay as one photon, mass cannot be created.

Could someone please confirm if this is true or false, and why? or what conditions must be met?

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  • $\begingroup$ Think about this: Once the photon is absorbed, there is some amount of energy stored in whatever absorbed it. Can this object release that energy in the form of two photons? $\endgroup$
    – Danu
    Commented Feb 4, 2016 at 15:49
  • $\begingroup$ So would it be releasing the two photons with lower energy? $\endgroup$
    – gptt916
    Commented Feb 4, 2016 at 15:55

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Absolutely it can - and it happens all the time. If you excite an atom, it can go through various "stages" of decay back to the ground state - with each drop in energy resulting in an emission of radiation.

This happens during photosynthesis: see this page from which I copy this image:

enter image description here

As you can see, there are multiple paths for the energy to be lost by various transitions - some of which involve the emission of light.

Note that in some multi-level systems, some of the transitions are non-radiative - but there is no reason why they have to be.

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    $\begingroup$ I see, so essentially an excited atom can decay at different rates, through which it radiates photons, but not necessarily the same energy as the absorbed photon? $\endgroup$
    – gptt916
    Commented Feb 4, 2016 at 16:12
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    $\begingroup$ As long as it emits no more energy than it absorbed, yes. Often some energy turns into vibrations (heat). $\endgroup$
    – Floris
    Commented Feb 4, 2016 at 16:21
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Yes, absolutely! This is the phenomenon of so-called non-linear light scattering. These include an absorption of a photon and its remission as several photons, or as a photon of different frequency, or a combination of a photon and a phonon, etc. (You may want to look up Raman scattering and Mandelstam-Brillouin scattering.)

Note also that the Rayleigh scattering of the photons with the same frequency does not actually involve the absorption.

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