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If we observe isolated chlorophyl solution with UV light, chlorophyl is seen as red. Explanation suggests that when a specific atom of chlorophyl absorbs UV light, the atom gets excited and in a short time electrons still goes from excited state to ground state. But why it is not emits same wavelength light, and instead emits lower energy light - red ?

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    $\begingroup$ Many materials do that. Search for fluorescence. $\endgroup$
    – Poutnik
    Nov 26, 2021 at 20:28
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    $\begingroup$ Besides generally searching for “fluorescence”, see here and perhaps search in this stack exchange for other answers. $\endgroup$
    – Ed V
    Nov 27, 2021 at 0:03
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    $\begingroup$ Very basic and quick - I think is what you are asking for. As molecules are constituted of different part linked and vibrating, the upwards path can be closed once absorption occurred. Part of the energy is dissipated in rearranging the molecule and by vibrations. Then from a certain already lowered down level, emission occurs. As a starting point for more look indeed at the thread linked by Ed V. Wikipedia on the subject is also nice. Note that "isolated"guarantee emission, it is not functional to the fact that UV in / red out. A molecule which is not isolated can have its emission quenched.. $\endgroup$
    – Alchimista
    Nov 27, 2021 at 12:14
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    $\begingroup$ ... because the surrounding can interact with it and remove the otherwise "to be emitted" energy. $\endgroup$
    – Alchimista
    Nov 27, 2021 at 12:17
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    $\begingroup$ @EdV indeed the "guarantee" I have used above is with the context given. OP: Isolated molecules can be not fluorescent, or doing it with a very small efficiency nevertheless. $\endgroup$
    – Alchimista
    Nov 27, 2021 at 15:52

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There are three types of re-emission when photons interact with matter:

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  • $\begingroup$ I think it will be better to call "emit same wavelength" condition as resonance fluorescence rather than reflection. Atoms show this phenomenon in flames. If we excite Na atoms in flame by 589 nm, they will emit 589 nm. $\endgroup$
    – AChem
    Nov 28, 2021 at 1:45
  • $\begingroup$ Upvoted, but it is not clear to me what is puzzling the OP. Is it the matter of the “chlorophyl atom” not behaving like a hydrogen atom, i.e., it can absorb UV light, but fluoresces red? Or is the red fluorescence itself the issue? I think the former is what they are asking about, but unless they clarify or elaborate, not much more to say, I guess. $\endgroup$
    – Ed V
    Nov 28, 2021 at 21:51

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