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snorkack
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Moderator's note: thread split off from https://www.physicsforums.com/threads/confusion-about-absorption-spectra-cool-gasses-absorb.964474/
A completely white body can neither absorb nor emit - no matter what its temperature may be.
And objects that possesses spectral lines are neither black nor white - they are coloured.
For example, consider warm He gas.
It possesses spectral lines of absorption... bluewards of 60 nm. Yes, you can cause it to radiate thermally... by making it hot enough to excite appreciable numbers of electrons. In that case, you also excite other transitions of lower energy, including visible, as well as absorb visible light.
But if He is not hot enough to excite electrons, then it has no spectral lines available. It possesses internal excitations of translational movement of atoms relative to each other, but these have no dipole moment and no way of emitting radiation.
So warm He is an example of heat, but no thermal radiation because no absorption.
Gases are not required to be black bodies.lomidrevo said:I don't get your point, thermal doesn't mean infrared! It is because the everyday temperatures we encounter here on Earth means that the thermal emission of the everyday objects peaks at infrared part of the spectrum. For hot stars the thermal radiation may peak in visible or UV part of spectrum. Btw. thermal radiation and blackbody radiation are basically synonyms (at least in astrophysics). So that is for the continuous spectrums...
And regarding discrete absorption lines, they are not restricted to visible part of the spectrum only. You can have lines in all parts of the spectrum (UV, IR, MW, radio...).
A completely white body can neither absorb nor emit - no matter what its temperature may be.
And objects that possesses spectral lines are neither black nor white - they are coloured.
For example, consider warm He gas.
It possesses spectral lines of absorption... bluewards of 60 nm. Yes, you can cause it to radiate thermally... by making it hot enough to excite appreciable numbers of electrons. In that case, you also excite other transitions of lower energy, including visible, as well as absorb visible light.
But if He is not hot enough to excite electrons, then it has no spectral lines available. It possesses internal excitations of translational movement of atoms relative to each other, but these have no dipole moment and no way of emitting radiation.
So warm He is an example of heat, but no thermal radiation because no absorption.
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