If black bodies absorb all the radiation and emit radiation by themselves, why don't they absorb their own emitted radiation? We shouldn't see the sun for example, since it's a black body.
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3$\begingroup$ You misunderstand the term "absorb" with respect to electromagnetic radiation. "Absorption " is is not an attractive force. The radiation emitted goes away from the object emitting it, does not turn around or stay around the object. It is incoming radiation that can be absorbed or reflected. $\endgroup$– anna vCommented Jan 6, 2023 at 13:13
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$\begingroup$ Closely related physics.stackexchange.com/questions/645671/… $\endgroup$– ProfRobCommented Jan 7, 2023 at 13:54
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1$\begingroup$ I see the difference between absorption and emission. But take the sun as an example. Atomic fusions in the core lead to photons being produced and these photons go through different layers of the sun to reach the photosphere and eventually be released into space. However, the layers that these photons go through don't absorb them. Why is that? $\endgroup$– Authentic MelodyCommented Jan 7, 2023 at 13:59
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4 Answers
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Black bodies absorb the radiation that hits them. They don't absorb radiation moving away from them (nor do they attract or influence such radiation in any way).
answered Jan 6, 2023 at 13:19
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An object that absorbs all radiation falling on it, at all wavelengths, is called a black body. When a black body is at a uniform temperature, its emission has a characteristic frequency distribution that depends on the temperature. Its emission is called black-body radiation.
So black body absorbs all radiation and in turn emits radiation according to black body radiation law. So photons emitted inside of black body volume may be re-absorbed and re-emitted multiple times (actually it will be a different photon, cause old one was absorbed, but for the sake of argument let's assume same photon goes through absorbance-emittance cycle). Such re-emit + re-absorb cycle until photon reaches body surface and escapes once and for all for a huge black body can be really long. For example typical photon generated at Sun core jiggles back-and-forth between particles for about $100~000~\text{years}~!$ until it leaves the Sun and is ready to go into outer space.
answered Jan 6, 2023 at 13:20
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Radiation incident on a surface is either absorbed, reflected or transmitted. A "black body" has negligible reflection and transmission (strictly speaking none, but real objects that don't quite manage this are often worth calling "black bodies" if only as an approximation). By Kirchoff's law, such bodies are also especially good radiators. And since their radiation isn't incident on the surface (it's heading in the wrong direction), it's neither absorbed nor reflected nor transmitted. So if it's hot enough to glow relative to its colder surroundings, a black body looks white (until something else makes it look another colour).
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$\begingroup$ @PM2Ring By white I meant "the version of white associated with its surface temperature", not "the version of white we evolved to see because we live in sunlight", but either definition has more of some frequencies than others. The Sun's surface temperature, of course, is 5778 K. (Our eyes require slightly warmer light for a "pure white" perception because we're extra-sensitive to green, but that's getting into the physics of overlapping opsin spectra.) $\endgroup$– J.G.Commented Jan 10, 2023 at 7:59
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why don't they absorb their own emitted radiation
Radiation is not marked for belonging to the black body or something else (not in the least, because photons are indistinguishable.) It is better to think of the radiation and the black body existing independently from each other, and the black body emitting just as much of radiation as it absorbs.
Moreover, one does not need a black body to have a black body radiation - the latter can be simply defined as radiation in thermal equilibrium.
The situation with Sun is a bit trickier: the radiation emitted as a result of thermonuclear reactions within the Sun core is absorbed and reemitted many times before it reaches the Sun surface. As a result, it exists as black body radiation within the Sun. The radiation emitted by the Sun is not, strictly speaking, black body radiation, since it clearly does not return to the Sun: it is easy to see that the Sun with surface temperature of about 6000K is not in thermal equilibrium with the surrounding vacuum at temperature just a bit above 0K. However, since only small part of the radiation existing in the Sun is emitted, it is permissible to describe it by the black body spectrum, which corresponds to the radiation state within the Sun.
For more discussion see thread How does radiation become black-body radiation?
