## Black body

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Benitosimies
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### Black body

I'm reading Introducing Quantum Theory (I like the pictures) to get my head around, uh, Quantum Theory.

There's some things I don't understand. Please correct me if I need it.

I'm up to the part with black bodies being awesome for measuring thermal radiation. If an oven has a little hole in it, radiation can get in there, but bounces all over the place onto the walls before coming back out. The walls of the oven absorb the radiation and then re-emit it.

QUESTION 0: How can you just straight-up say that things 'absorb' and 'emit' radiation?

When the walls suck up more radiation, the radiation they emit starts to enter the visible spectrum (starting with red, orange, yellow...) and you can say that you can see the frequency of the waves in the oven. As the oven gets more crowded with waves, the wavelengths get shorter (and the color therefore moves towards greens and blues and finally ultraviolet). +This doesn't actually happen, I think what happens is that energy decreases as frequency increases, after a while (see the COBE graph)+

QUESTION 1: Does a black body have to have a hole leading to a cavity for it to be considered a black body? Could I say a black slab of iron is a black body?

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sdedeo
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### Re: Black body

Benitosimies wrote:QUESTION 0: How can you just straight-up say that things 'absorb' and 'emit' radiation?

I'm not sure I understand the question here. To "absorb" radiation, it means you take the energy in the electromagnetic field of the light and dump it in to some other system (e.g., into the motion of electrons in the slab.) And "emission" is simply the reverse. In general, the "trick" to the black body is that all these processes increase (or maintain) the level of entropy. You could imagine a bizarre cavity that absorbs radiation at all frequencies, but emits it only at one particular frequency; this would not be a black body; technically speaking, it has an emissivity that is different from unity.

That's a bit circular, perhaps, but it is not a tautology. The idea of a system absorbing light usually means dumping that electromagnetic energy into a large reservoir (the electron sea of the iron, e.g.) that loses all memory of the incoming frequency and radiates in thermal equilibrium.

Benitosimies wrote:QUESTION 1: Does a black body have to have a hole leading to a cavity for it to be considered a black body? Could I say a black slab of iron is a black body?

Short answer: yes. The reason you have a "cavity+hole" is that it means that even if the surface on the inside has some reflectivity, the light will still bounce around enough that it will get absorbed before it finds the hole and escapes. If you just have a slab that's really not reflective at all, it's a good approximation.

Benitosimies
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### Re: Black body

sdedeo wrote:
I'm not sure I understand the question here. To "absorb" radiation, it means you take the energy in the electromagnetic field of the light and dump it in to some other system (e.g., into the motion of electrons in the slab.) And "emission" is simply the reverse.

Okay, that helps a lot. I should have been keeping in mind Thermal Equilibrium.
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The Ethos
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### Re: Black body

Do blackbodies really have to have a cavity? A blackbody emitter doesn't necessarily need to be hollow. Is this a different version of the same word?
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Micron
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### Re: Black body

The Ethos wrote:Do blackbodies really have to have a cavity?

No.

If it wasn't clear I believe sdedeo was responding "yes" to "Could I say a black slab of iron is a black body?"

A cavity with a small hole in it is a good way to experimentally approximate an ideal black body. Using a cavity means you have very little reflection off the surfaces of the object and so the emitted light is almost entirely radiated. You can use an object of any shape but it will reflect some of the light that hits it which might distort it's apparent radiation.

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### Re: Black body

The Ethos wrote:Do blackbodies really have to have a cavity? A blackbody emitter doesn't necessarily need to be hollow. Is this a different version of the same word?

It's a question of how "black" do you want it to be, and at what wavelengths. A true, perfect blackbody is a blackbody at all wavelengths (absorbs everything that hits it, re-emits in a thermal distribution corresponding to its physical temperature), but in many cases you don't need it to absorb absolutely EVERYTHING, nor do you care what it does at wavelengths that you're not interested in. For example, a typical laboratory blackbody (hot cavity with a hole in it) will be a good IR/visible blackbody but probably not so good at, say, radio frequencies because the hole will act as a waveguide high-pass for suitably large wavelengths. Similarly, at mm-wavelengths, we often use absorber materials that are ~99% absorptive in the desired band but would likely not at all be "black" at visible wavelengths -- but that's often good enough.

eternalfrost
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### Re: Black body

a "blackbody" absorbs 100% or radiation that touches it, none is reflected(where the 'black' part comes from), or passes through it. this energy goes into heat within the body.

absorption and emission properties go hand in hand. your perfectly absorbing body will also emit perfectally, getting the gaussian shaped emission curve depending on the temperature of the body

in practice, there arent any true black bodies. howver, the 'cavity with a pinhole' model is about as close as you can come. the idea is that energy entersthe hole(say you beam a laser in at an angle). now virtually any light that gets reflected instead of absorbed stays within the cavity. in this way, nearly none of the original light is reflected off the body,only infrared heat being emitted as the object heats up, qualifying it as a blackbody of sorts

DarkLordofSquirrels
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### Re: Black body

The name's a bit ambiguous. The Sun is very close to being a black body, as the radiation it reflects is negligible compared to that which it emits properly.

Ashbash
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### Re: Black body

Even if you have a ridiculously massive light source to shine at it?

[I'm actually trying to understand, black bodies is the only thing I don't properly understand so far in physics (high school)]

Owehn
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### Re: Black body

It isn't the total amount of light that reflects; it's the percentage. If you shine a ridiculously bright light at the sun, most of it will still be absorbed and re-emitted as (approximate) blackbody radiation, and only a small proportion will be reflected.
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SpitValve
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### Re: Black body

Benitosimies wrote:+This doesn't actually happen, I think what happens is that energy decreases as frequency increases, after a while (see the COBE graph)+

Photons with higher frequencies have higher energies. Always. E=h*f

The COBE graph is an almost perfect black body spectrum (better than the sun's chopped up one) that has come to us from the instant the universe became transparent. Really really hot things tend to be good black bodies, and the entire universe was hotter than the sun's surface at that point. The Cosmic Microwave Background doesn't look very hot though, because the universe has expanded and stretched the photons into lower wavelengths.

A spectrum tells you how many photons are at each wavelength/frequency/energy. So there aren't that many photons in the CMB with really high or really low energies.

Minerva
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### Re: Black body

There are a great many things that we can consider to be approximately blackbodies, at least over a certain waveband that we're interested in.

Stars can be treated as approximate blackbodies, as can planets, heated chunks of steel, incandescent lightbulbs, and so forth.

In fact, if you're only looking at some waveband, and you're aware that it's only a non perfect approximation, you can consider nearly anything to be a black body for certain purposes.
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