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A blackbody is a theoretical object that perfectly absorbs all the light that falls on it. From what I understand this is an ideal situation and does not actually exist in reality. Certain objects are close to being a blackbody but they do not absorb 100% of the light that hits it (i.e. some gets reflected).

I have seen a few articles refer to a blackbody as a "perfect blackbody" or an "ideal blackbody". Isn't it redundant to describe a blackbody as being perfect or ideal? The defining property of a blackbody is that it completely absorbs all light that hits it.

So this is how I understand it: a blackbody is a perfect absorber and all other object that aren't perfect absorbers of light are close-blackbodies or near-blackbodies. I found the following quote online and the terminology contradicts what I have read and understood from other sources. If anyone knows which one is actually correct can you please clarify it for me.

A perfect blackbody absorbs all wavelengths of incoming electromagnetic radiation perfectly; none of it is reflected. A perfect blackbody at a given temperature emits radiation in accordance with Planck's law. Any real blackbody is imperfect in both of these respects; it does not absorb all incoming radiation, and it does not have an emission spectrum that perfectly matches Planck's law.

A blackbody is also a good emitter of light. If this was not true, water placed next to the sun would freeze instead of evaporating away. Logically, this makes sense because the sun absorbs the energy around it including energy stored in the water. This isn't what we observe in real life so energy must be emitted from the sun back into its surroundings. I just don't understand why this has to be the case. Theoretically, why couldn't the sun just absorb the surrounding energy without emitting it back? In a similar fashion to black holes (this idea could be totally wrong, I don't really know much about black holes). Ultimately my questions is why do good absorbers have to be good emitters too?

As a side note, would it be correct to call this system (the sun absorbing energy from its surroundings) endothermic?

All objects above absolute zero emit radiation. The amount of radiation is dependent on the object's temperature. Do we call this radiation blackbody radiation? This is where I am really confused. I thought blackbody radiation was electromagnetic radiation emitted by a blackbody a theoretically perfect absorber and emitter.

What would be some examples of objects found in nature that are very close to being a blackbody? So far I have stars/our sun and carbon black.

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  • $\begingroup$ star and sun are not black bodies. $\endgroup$ – Anubhav Goel Nov 21 '15 at 12:26
  • $\begingroup$ A perfectly shiny hollow metal sphere is also a black body. It's not just a matter of absorptivity, but also of uniformity across wavelength. No light can escape from the interior, so sooner or later every photon gets absorbed (and photons are emitted, to maintain the thermal equilibrium). $\endgroup$ – Carl Witthoft Nov 21 '15 at 15:19
  • $\begingroup$ Also, every body is equally good at emitting and absorbing radiation from the surroundings, otherwise it would be at a temperature different from the surroundings at equilibrium, violating the law of thermodynamics. $\endgroup$ – Harsha Nov 21 '15 at 20:34
  • $\begingroup$ @AnubhavGoel : star and sun ARE pretty good black bodies, in the sense that their emission is well approximate by Planck spectrum. Only stars like geant blues are not, because of the accumulation of peaks in the spectrum. $\endgroup$ – Fabrice NEYRET Nov 22 '15 at 1:20
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Is the Sun absorbing energy from it's surroundings? No, of course not in a net sense. The Sun loses far more energy than it absorbs from its surroundings. It is not in thermal equilibrium.

The Sun is also not a blackbody at a single temperature, even though it most definitely absorbs nearly all radiation that is incident upon it. That is because the Sun is not at a single temperature and we see to different depths and different temperatures at different wavelengths. The Sun is not all at the same temperature because it is not in thermal equilibrium.

I suspect it may be this aspect of blackbody radiation that you have missed and which is present in the first sentence of the relevant wikipedia page.

As for other examples of blackbodies - well, the inside of a kiln as viewed through a small hole would be quite reasonable it if has been left long enough to achieve a reasonably uniform temperature.

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The best representative of a black body curve is the cosmic microwave background radiation.

cmb

Graph of cosmic microwave background spectrum measured by the FIRAS instrument on the COBE, the most precisely measured black body spectrum in nature. The error bars are too small to be seen even in an enlarged image, and it is impossible to distinguish the observed data from the theoretical curve.

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Ultimately my questions is why do good absorbers have to be good emitters too?

For a body in radiative equilibrium, absorption and emission should balance, by definition of equilibrium. Why this happens, because the complex quantum mechanical levels that absorb the electromagnetic radiation are also the ones who will emit it, falling back to a lower energy state.

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