So,in my physics school book it’s written that atoms don’t radiate as long as they are stable ”in the ground state” I can’t understand how I mean isn’t all the bodies radiate as long as they have a certain temperature is all the atoms in the universe are unstable?
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Also have a look at What are the various physical mechanisms for energy transfer to the photon during blackbody emission? though if you're still at school the answers there may be a bit too complicated for you. – John Rennie Mar 29 '22 at 15:52
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Atoms in a material are only in their ground states with probability $1$ when $T=0K$, at a non-zero temperature they will have some probability of being in an excited state and so can radiate by spontaneous emission. So in theory if you cooled something to absolute zero it should cease to radiate entirely, but this is only because the atoms constituting the body are stable and don't decay.
If this didn't answer your question please leave a comment.
AfterShave
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Ι mean isn’t all the bodies radiate as long as they have a certain temperature
Atoms and single particles in general are not a "body". The "bodies" that radiate are composed out of about $10^{23}$ atoms per mole, and temperature is a thermodynamic variable defined over a large number of atoms/molecules .
A single atom is a quantum mechanical entity, with electrons in bound states with the positive nucleus, and is stable and will only radiate if it absorbs and reemits a photon, with specific energy allowed by the quantum mechanical solution. An atom does not have a temperature
Radiation from solid masses of atoms only approximately obeys the black body radiation curve, to the extent that the vibrational and rotational levels of the collection of atoms radiate.
See the approximate black body radiation from the sun measured at the top of the atmosphere, the curve for 5178K (yellow curve)
It has the effect of the spectra of various atoms exciting and de-exiting , plasma interactions too, approximately fitting the black body theoretical formula.
The radiation of the atmosphere at 294K is even a worse fit due to the spectral emissions.
Outgoing spectral radiance at the top of Earth's atmosphere showing the absorption at specific frequencies and the principle absorber. For comparison, the red curve shows the flux from a classic "blackbody" at 294°K (≈21°C ≈ 69.5°F).
anna v
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Isn't that spectrum's deviation from a blackbody curve a result of absorptive and emissive processes in the sun and the earth's atmosphere, not the initial blackbody source(s) failing to abide by Planck's law? – g s Mar 29 '22 at 19:51
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@gs the yellow curve is before the atmosphere. the yellow is what happens before leaving the sun. Yes, all sort of absorptive and radiative processes of the plasma are at work, including atomic ones, I just give it as an easy example of the approximate bb form – anna v Mar 30 '22 at 04:13
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@gs In the case of the Sun, deviations from blackbody occur because the radiation is coming from layers with different temperatures. The solar atmosphere is more opaque at atomic resonances, and so the radiation at those wavelengths comes from a higher, cooler layer. Thus, we see Fraunhofer lines (https://en.wikipedia.org/wiki/Fraunhofer_lines) in the spectrum. Additionally, in the case of the Earth, there are molecular resonances and the ground isn't black, so at wavelengths where the atmosphere is transparent, the emission is less. – John Doty Mar 30 '22 at 15:24
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Atoms don’t radiate as long as they are stable ”in the ground state”
Yes.
isn’t all the bodies radiate as long as they have a certain temperature
Yes.
is all the atoms in the universe are unstable??
No.
When excited atom emits photons, after some time it will return to a ground state and be ready for next excitation. In ordinary materials most atoms are in ground state at particular time moment. In contrary, most atoms in laser active material are in excited state, due to that it's called population inversion. Whether or not system is in normal or inverted state, atom behavior does not change,- typical atom juggles between excited and ground states from time to time. Though probabilities of finding n-th atom in excited or ground state can differ as well as probabilities of specific transitions to ground/excited or meta-stable levels.
Agnius Vasiliauskas
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