We know that we don't have perfect black bodies around us, we also have plots of spectral distribution of energy in case of black body, but what happens in real life cases? Do we as well emit radiation of all wavelength following Stefan's and Wein's law. My problem is that how does actual bodies have their spectral distribution of energy and if it is same as that of blackbody, then can we say that blackbody is just another body which absorb all radiation at thermal equilibrium and nothing significantly different properties?
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asked Jan 16, 2020 at 6:52
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$\begingroup$ info: The best fit to thablack body radiation formula comes from the CMB en.wikipedia.org/wiki/Cosmic_microwave_background#Features $\endgroup$– anna vCommented Jan 16, 2020 at 8:39
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3 Answers
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We humans, and all other bodies that are above absolute zero, do radiate energy- but note that your skin is not perfectly black: it has a certain color. Right there you know that your spectral emission isn't going to be a good match to that of an idealized black body; there will necessarily be hills at certain wavelengths and valleys at others. No big surprise there.
Also note that one of the basic assumptions of the blackbody law is that the body is both a perfect absorber and a perfect emitter, at all wavelengths. This is not always true; there are for example engineered surfaces which have been optimized for use as solar energy collectors which are better absorbers than they are emitters at various wavelengths.
Neither of these facts invalidate the correctness of the essential physics at the root of the blackbody law, so do not lose hope!
answered Jan 16, 2020 at 7:29
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$\begingroup$ In the thermal infrared, all humans are black. $\endgroup$– user137289Commented Jan 16, 2020 at 8:02
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$\begingroup$ in the nearby visible, only some are. $\endgroup$ Commented Jan 16, 2020 at 18:06
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$\begingroup$ ...and at at night, of course, all cats are grey. $\endgroup$ Commented Jan 16, 2020 at 22:16
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$\begingroup$ This is not correct: "your spectral emission isn't going to be a good match to that of an idealized black body". The match will be excellent, even for the whitest WASPs. $\endgroup$– user137289Commented Jan 16, 2020 at 22:23
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$\begingroup$ but Pieter! why then does my skin not appear black to my eyes? Why not post your own answer- and I'll delete mine. -NN $\endgroup$ Commented Jan 17, 2020 at 0:00
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Bodies with a temperature above 0 does emit radiation according to the blackbody - theories. However, we do not have emissivity- or absorbtivity-values of $1$ but instead we're modelled with values between $0$ and $1$. For example, the planets in our solar system can with high accuracy be described with blackbody theory.
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Yes, objects around room temperature emit radiation. It is easy to estimate the peak wavelength with Wien's law: the spectrum of the Sun has its maximum around $0.5\ \mu$m at a temperature of about $6000$ K. Our temperature is about $300$ K so the peak wavelength will be twenty times longer, around $10\ \mu$m.
This is in the thermal infrared, long-wavelength infrared. Almost all materials are strongly absorbing in this region of the spectrum because these are frequencies that match molecular vibrations. Such AC electrical fields will excite heteronuclear molecular bonds and the other way around.
The only common exception is metals. Those are reflecting.
This is why thermal cameras work. They collect the emissions around $8 - 12\ \mu$m and can convert to temperature without too many worries about different emissivities. Generally those are about $0.9$ or higher. I would not expect tattoos to be visible in thermal imaging.