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I am asking this regarding blackbody radiation. Water is not black, obviously. I read it is very lightly blue, but it is mostly transparent in liquid form so it probably does not absorb much of the visible spectrum nor emit anything at least at low temperatures. In solid or gazeous form it is mostly transparent too. However, if water is heated to extremely high temperatures, whatever its phase and pressure, can it produce light?

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  • $\begingroup$ Well, any atom or molecule will fluoresce when excited, but the fluorescence lifetime may be far too short (or of non-visible wavelengths) to be observable. $\endgroup$ Sep 11, 2020 at 13:08
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    $\begingroup$ Note that thermal radiation is not luminescence, so either the title doesn't match the question body, or I don't understand the question correctly. As for luminescence, there's a phenomenon known as sonoluminescence, which does happen in water. $\endgroup$
    – Ruslan
    Sep 11, 2020 at 13:26
  • $\begingroup$ @Ruslan, I modified the title. I know sonoluminescence and that is not where my interest lies with this question. $\endgroup$
    – Winston
    Sep 16, 2020 at 3:54

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Water (in more or less normal conditions) has just a tiny "window" of transparency at and around the visible light.enter image description here (image from https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water)

One can pretty much feel the thermal infrared radiation near a hot (sub-boiling) water.

In order to see significant visible light emitted from water, you have to heat it a lot. In particular, burning a hydrogen in air does not produce enough heat for a flame to be visible and this is considered a danger. If you have enough line-of-sight inside the flame and it is dark enough, some tiny light is visible (by eyes).

Then again, given enough heat, water will decompose and ionize. A plasma with enough free electrons is pretty good at emulating a blackbody.

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  • $\begingroup$ Thanks. Will water have to decompose into oxygen and hydrogen before it emits visible light or can it do it at lower temperature? What temperature would be enough to start emitting visible light and what color would that be, the same it absorbs? $\endgroup$
    – Winston
    Sep 16, 2020 at 3:51
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    $\begingroup$ No, you don't need to decompose it. At ~500C /800K everything starts to glow visibly. At that temperature the water is perfectly stable against thermal decomposition and it is stable enough all the way to, say, 2500K. And yes, the emission specter for small optical depths should be something like blackbody multiplied by absorption specter. $\endgroup$
    – fraxinus
    Sep 16, 2020 at 9:27
  • $\begingroup$ p.s. I meant "inverted absorption specter" in the above comment! $\endgroup$
    – fraxinus
    Sep 16, 2020 at 9:48
  • $\begingroup$ No problem, I understood what you meant. $\endgroup$
    – Winston
    Sep 16, 2020 at 9:55

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