Talk:Black-body radiation

From Wikipedia, the free encyclopedia
Jump to: navigation, search
WikiProject Physics (Rated C-class, Mid-importance)
WikiProject icon This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
C-Class article C  This article has been rated as C-Class on the project's quality scale.
 Mid  This article has been rated as Mid-importance on the project's importance scale.
 
WikiProject Astronomy (Rated C-class, Low-importance)
WikiProject icon Black-body radiation is within the scope of WikiProject Astronomy, which collaborates on articles related to Astronomy on Wikipedia.
C-Class article C  This article has been rated as C-Class on the project's quality scale.
 Low  This article has been rated as Low-importance on the project's importance scale.
 

Note[edit]

This page was transferred from Black body. For discussion of this topic prior to this transfer, see Talk:Black body. Brews ohare (talk) 18:02, 24 January 2012 (UTC)

Comment[edit]

Hmmm... I'm not certain this is an improvement. It is true that if you could sample the radiation being exchanged between two bodies (say) that are in thermal equilibrium with each other and their surroundings, you'd get a black body spectrum. But most sources of approximate BB radiation that we see in nature or the lab aren't like that. Instead, you typically have a single localized body, for instance a cavity with a small hole in it, or the sun, that is not in equilibrium with its surroundings (although it in in equilibrium with itself, and the rate it is losing energy by radiation must be slow for the BB spectrum approximation to hold). Such bodies emit radiation, but don't absorb much (one important exception is the cosmic microwave background radiation, which really is a sample of the radiation "inside" the body).

For a situation where the body is isolated and emitting but not absorbing it's not true that any body will emit the Planck spectrum - only a black body (a perfect absorber) will. The way the article is written now, I don't think a reader would understand that. Waleswatcher (talk) 18:11, 24 January 2012 (UTC)

Another comment/question: what distinguishes the Planck's law article from this one? Waleswatcher (talk) 18:16, 24 January 2012 (UTC)

I haven't looked into the content of Planck's law or Thermal radiation. It may be that separate articles are not useful.
I changed the introduction a bit. If you think it should be reworded, go for it. It may be that the provided definition is adequate, but some practical realizations should be mentioned.
I do believe that separate articles for black-body radiation and black body is useful. They are separate concepts, since a black body simply as a perfect absorber needn't be in thermal equilibrium. Brews ohare (talk) 19:33, 24 January 2012 (UTC)

Dick Lyon: Sum of the emitted and reflected radiation power ?[edit]

Hi Dick: Your edit here seems confusing to me. The sentence you have written is:

At equilibrium, of course, the amount of radiation emitted by a body at each frequency is the same as that absorbed; for black-body radiation, at each particular choice of frequency the sum of the emitted and reflected radiation power is determined entirely by the equilibrium temperature.

Can you explain to the reader (i) what is "radiation power" and (ii) if the amount of radiation emitted at each frequency is the same as that absorbed, then why specify the sum as dictated by the temperature, when each term individually is the same? Brews ohare (talk) 22:02, 24 January 2012 (UTC)

I agree, it's still lame. By "radiation power" I meant the same thing as "amount of radiation"; feel free to change. The bit about equal amounts absorbed and emitted is true, but less relevant than the fact that the radiation (emitted plus reflected) is what depends only on temperature. What it said before about emitted would only be true if the emitter was a black body, which kind of misses the whole point of talking about equilibrium. It's the same redundancy confusion as in the black body article: you can get black-body radiation either from a black body not in equilibrium with its environment, or from any body in equilibrium with its environment. Specifying both is just weird. Dicklyon (talk) 22:09, 24 January 2012 (UTC)
Is it possible, then, that what you want to say in simpler terms is:
At equilibrium, of course, the amount of radiation emitted by a body at each frequency is the same as that absorbed; at each particular choice of frequency the sum of the amounts of radiation emitted and reflected (that is, the net amount leaving the surface, called the spectral radiosity) is determined entirely by the equilibrium temperature.
Brews ohare (talk) 22:15, 24 January 2012 (UTC)
I've taken another shot at this introduction; please take a look. Brews ohare (talk) 23:26, 24 January 2012 (UTC)

Waleswatcher: emitted and absorbed radiation are exactly equal[edit]

This edit of yours is based upon the edit summary that titles this thread. I believe it is based upon a misreading of what was said. The replaced text is:

At equilibrium, at each particular choice of frequency the sum of the amounts of radiation emitted and reflected from a body (that is, the net amount leaving its surface, called the spectral radiance) is determined entirely by the equilibrium temperature, and does not depend upon the shape, material or structure of the body.[Ref 1] For a black body, none is reflected, of course, so the spectral radiance is determined entirely by emission. In addition, a black body is a diffuse emitter (its emission is independent of direction). Consequently, black-body radiation may be viewed as the radiation from a black body at thermal equilibrium.
Reference
  1. ^ Joseph Caniou (1999). "§4.2.2: Calculation of Planck's law". Passive infrared detection: theory and applications. Springer. p. 107. ISBN 0792385322. 

I want to point out that this text refers to the sum of reflected and emitted radiation and, unlike your one-line edit comment, does not mention absorbed radiation. This text refers to any body, not to only a black body. Moreover, this text is important in indicating how the term black-body radiation becomes attached to radiation in thermal equilibrium with the walls of its container, something less than obvious.

Waleswatcher, unless you have some substantial reason opposing the change, this text should be restored to its original form, with the reference that makes it all clear. Brews ohare (talk) 02:21, 25 January 2012 (UTC)

I'm sorry - I did in fact misread that. I withdraw my objection to its content, but I think it needs to be re-worded as it's not very clear. I'll try to do so. Waleswatcher (talk) 02:45, 25 January 2012 (UTC)
OK, I restored a reworded version. Please see if you like it, and also if you agree with me that it should be moved out of the lead and down somewhere in the body of the article. I think it's a bit too technical for a reader that just wants a summary. Waleswatcher (talk) 02:51, 25 January 2012 (UTC)
Thanks for the change. It's not perfect, but this is WP, eh? Brews ohare (talk) 03:50, 25 January 2012 (UTC)

first sentence having it each way[edit]

At present the first sentence reads: "Black-body radiation is the electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or that is emitted by a black body that has a temperature uniform in space and constant in time."

I think this is an invitation to muddle. It seems to me that only under very exceptional circumatances can a body have a temperature that is exactly uniform in space and invariant in time and not be thermally irradiated by an environment with the same temperature. True, it can be approximately so under non-exceptional circumstances. There is nothing wrong with stating an approximation.

But if one wants to talk about approximations, I think one should say so explicitly. Since the first sentence does not say explicitly that in the second principal clause it deals with an approximation, then it should mean that it does not deal with such an approximation. In that case, the second principal clause simply repeats in loose language the proposition of the first principal clause. The obvious conclusion to me is that the second principal clause is redundant and loosely worded and is really an invitation to muddled thinking. I would prefer to delete the second principal clause, and make the sentence read simply Black-body radiation is the electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment.

If it is desired to say that very soon after the body has been removed from an environment of perfectly uniform temperature it will have a very nearly uniform temperature, and will emit very nearly Planckian radiation, then it should be so said explicitly in a separate sentence, that makes it clear that it is talking about what is very nearly so.Chjoaygame (talk) 14:17, 27 January 2012 (UTC)

I understand what you're saying, but I don't agree that clause should be deleted. The reason is that at large fraction of the time black body radiation comes up in applications, it's to an object that's at some temperature - usually not equal to that of the environment around it - that is radiating. Examples include the sun, objects viewed with night-vision equipment, glowing pieces of hot metal, etc. And much of the rest of the article deals with such objects. So I think the first sentence or so of this article must address that.
I do agree that it's confusing as written, but I don't think simply deleting it is a good idea. Waleswatcher (talk) 15:43, 27 January 2012 (UTC)
Well if you don't want to delete it, what about my second option: Make it a separate sentence and say it is a very very very very very very very very very good approximation!?Chjoaygame (talk) 16:00, 27 January 2012 (UTC)
Something like that, but not necessarily even talking about approximation yet. Define what the radiation is in terms of thermodynamic equilibrium. Then say that you also get it from a perfect blackbody at constant temperature; introduce the concept of approximation later in the article. Not so complicated. Dicklyon (talk) 16:36, 27 January 2012 (UTC)
Thermal equilibrium is the key to black-body radiation. I can't avoid the feeling that you are trying to set up a position in which you can make out that such is not the case. I feel you have a slogan to defend. You must be able to say "Wikipedia says I can say without reservation 'Such and such is black body radiation'." Why are you so keen to avoid the lead saying it is an approximation? Mitt Romney posted his income tax return. What about your approximation tax return? I think you owe us that.Chjoaygame (talk) 18:47, 27 January 2012 (UTC)

I removed some accurate and correct but potentially confusing or distracting (to a non-physicist) adjectives from the lead. I also re-attached the phrase about black body emission to the lead sentence. I think it belongs there, because the majority of the time people care about BB radiation it's because something hot is radiating it into the environment, not because we've somehow opened a window into the interior of some system in perfect equilibrium. While the latter is also very important (and is even observable, for instance the CMB), the former belongs right at the start. Hope it's OK. Waleswatcher (talk) 20:16, 28 January 2012 (UTC)

black holes[edit]

It's actually not the case that spinning black holes are near-ideal black bodies, and by the same token their Hawking radiation is not BB radiation (it's not isotropic, and it's polarized). We can fudge that in the lead by deleting all mention of charge and spin, and then mention it later in the article. Waleswatcher (talk) 15:45, 27 January 2012 (UTC)

Why fudge? A black hole deserves an article of its own. A few sentences here cannot do it justice. It seems to me that you are right that will not be a strict black body radiator. The association black body/black hole sounds to me like a clang association, not really proper physics. I think the section should be just deleted.Chjoaygame (talk) 15:56, 27 January 2012 (UTC)
Because it's the lead - it shouldn't have too many details. Stated that way it would still be accurate, just incomplete. The charge and spin can be mentioned in what follows below. It's not true that this is a "clang association" (assuming I'm interpreting that phrase correctly). Black holes really are extraordinarily good black bodies, probably the best in the entire universe, so they deserve a mention. It's just that with very high spin there's a "grey body" factor that depends on the angular momentum of the radiation (similar to how for any "BB" of finite size, it won't absorb long wavelength radiation perfectly). Further, spinning black holes are kind of squashed in shape, and their Hawking radiation isn't isotropic as a result. Waleswatcher (talk) 16:53, 27 January 2012 (UTC)
Sorry, my mistake. Please ignore that comment of mine. I didn't read carefully, and didn't see we were referring to the lead. (Still, I have to admit I am sceptical, mainly from ignorance. But now I learn from the article on Hawking radiation that "For a black hole of one solar mass (M_{\odot} = 1.98892 × 1030 kg), we get an evaporation time of 2.098 × 1067 years—much longer than the current age of the universe at 13.73 ± 0.12 x 109 years." In my ignorance, I have in the past thought that black holes were normally very heavy objects, much more massive than the sun. Now I get the idea that it would be hard to check their radiation by measurement if that were so.)Chjoaygame (talk) 18:37, 27 January 2012 (UTC)

History[edit]

The history section is divided into parts titled "Stewart" and "Kirchhoff". That seems a bit odd considering the important role of black-body radiation in the formulation of quantum mechanics, which should lead at a minimum to sections titled "Planck" and "Einstein". Maybe a section titled "Wien"? Maybe something about experimental verification? Perhaps the sections should be recast to have titles referring to the key concepts, not to particular figures? Brews ohare (talk) 14:01, 31 January 2012 (UTC)

"Temperature relation between a planet and its star" needs to be scrubed of irrelevancies[edit]

The page is about black body radiation . The greatest excursion it should make from that is perhaps to note that radiantly heated gray , ie , flat spectrum , balls come to the same temperature as that calculated for black bodies . That is the temperature corresponding to the energy density at the location . In our orbit , that is approximately 279 kelvin which is about 3% less than our observed approximately 289 .

Any calculation or speculation about the color , non-flat spectrum , of the earth should be removed .

As it stands , the section on the Temperature of Earth is an embarrassment to Wikipedia . Bob Armstrong (talk) 05:14, 20 June 2012 (UTC)

Bob Armstrong, I agree with you about this. I am not keen even on the talk about the grey body, and think it probably better omitted. I trust you have a good plan to do the scrubbing and will go ahead with it at your leisure.Chjoaygame (talk) 08:18, 22 June 2012 (UTC)

Breathing always leads to water loss[edit]

Can someone check if the missing citation on the human body emission can be covered by this link: http://www.ncbi.nlm.nih.gov/pubmed/16550955? Also, I don't know how to create the citation entry. Thanks. Marco 17:40, 24 July 2012 (UTC)

I would say that this reference is not suitable for the purpose. It is about specialized basic research. Such is not the preferred kind of reference. The preferred kind of reference would be from an up-to-date and well respected textbook of human physiology, with a pointer to pages that give a general account of heat loss by evaporation in the lungs during breathing.Chjoaygame (talk) 01:22, 25 July 2012 (UTC)

undid edit that added reference[edit]

I have undone this edit because I am concerned that it appears to be part of a program of promoting the writings of a particular author, and may, for all I know, be an act of self-promotion by that author, in a case of sock-puppetry. Perhaps I am mistaken, but I would like some direct and explicit assurance or evidence that I am indeed mistaken in seeing that appearance.Chjoaygame (talk) 10:51, 18 April 2013 (UTC)

Edit suggestion for section: Explanation[edit]

The section titled "Explanation" begins referring to an "oven" ("A graph of the amount of energy inside the oven. . . ") but doesn't explicitly establish what that means. The reader can use context to infer that it means "A closed box of graphite walls at a constant temperature with a small hole on one side. . . ", which was stated earlier in the section, but the reader shouldn't have to infer that in my opinion. I suggest clarification in the paragraph that defines this closed box of graphite to name it an "oven". --24.11.245.121 (talk) 03:10, 26 July 2013 (UTC)

Good suggestion. Thanks. Done.Chjoaygame (talk) 09:43, 26 July 2013 (UTC)

undid new section[edit]

I have undone an edit that added a new section. I see several problems with the edit that I undid.

First, it is only marginally notable. Second, it makes assumptions that it does not explain at the level appropriate for this article. The assumption is that the body is black, yet it is proposed to be unfeasible to do the ordinary measurements for a black body. Third, the edit seems perhaps to be a self-promotion, by an editor who seems only to have made this one edit. Some kind of evidence that it is not self-promotion would be needed, I think. Fourth, the source is an arXiv paper. This is primary research, not even peer reviewed. This is in general not an adequately reliable sourcing according to Wikipedia policy.Chjoaygame (talk) 14:27, 8 January 2014 (UTC)

approximation versus estimate[edit]

The word approximation restored here by Editor 2/0 seems to me a little pretentious. Approximation for me carries the meaning of systematic gradual approach to exactitude. For me, the word estimate is vaguer and less pretentious. In this context, I would say that the black body model would be a zeroth order approximation, but I guess that the extra words 'zeroth order' are also too elaborate and pretentious for this context. That makes me prefer 'rough estimate'. Editor 2/0 agrees that the words have different meanings, but he doesn't spell out what he sees as the difference. Perhaps he will do so here.Chjoaygame (talk) 00:43, 27 July 2014 (UTC)

"radiation peak in the IR"; interesting edit unsourced; a source would be valued; and a question[edit]

An edit here has an edit cover note that reads "A barely visible black body has a radiation peak in the IR at a couple of microns".

Thank you for this interesting edit. A source would be good. Is the peak at a couple of microns only with respect to the wavelength or also with respect to the frequency?Chjoaygame (talk) 06:16, 3 December 2014 (UTC)

topic of this article[edit]

I would like to see two major sections of this article removed from it. I refer to the sections entitled 'Human body emission' and 'Temperature relation between a planet and its star'. I think those are both separate topics, only partly derivative in concept from the topic of the article, black body radiation. They are each important topics, and I am not here criticizing their contents: I am just saying that they are given far too much prominence in this article, when they should be prominent somewhere else.Chjoaygame (talk) 22:03, 29 April 2015 (UTC)