Why did scientists study black body radiations from something as complicated as a hollow container rather than the radiation from something simple like a thin solid cylinder?
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$\begingroup$ Unclear what you're asking and primarily opinion-based. To the others: Don't waste your 30 votes on this question. Flag/VTC it instead. $\endgroup$– Abhimanyu Pallavi SudhirCommented Aug 20, 2013 at 15:26
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$\begingroup$ This question just came up while i was studying and if you don't have any idea about it, just say so, I'll understand... $\endgroup$– vppCommented Aug 20, 2013 at 15:39
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$\begingroup$ Um, no. If I wouldn't have an idea about it, I would just leave it as it is, maybe vote upp, and not comment on it . It is your opinion that the latter is simple. And really very unclear what the context is . I'm convinced that I probably should downvote, as this "question" was actually upvoted ! . $\endgroup$– Abhimanyu Pallavi SudhirCommented Aug 20, 2013 at 15:49
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$\begingroup$ I'm hesitant to vote this closed. It would be helpful if you could provide motivation for the question. Also, given that the studies on black-body radiation were done a long time ago, are extremely accurate, and that no matter which way would be better it makes no difference now, are you purely interested in physics history or is there a sub-textual (perhaps unconscious) effort to point out that you could do the same thing but simpler and are, thus, smarter than the physicists who did the original work? Not implying anything, just sincerely curious. $\endgroup$– JimCommented Aug 20, 2013 at 18:30
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$\begingroup$ I don't mean any of that. I just came across this question in a physics hand book and was curious to know the answer(as there was no answer given). I'm not light headed enough to think that I could outwit any of the physicists who did the original work. For that matter I don't even know how a solid cylinder is simpler than a hollow cylinder, as you might have to take into account the internal absorption and reflection of incident light by individual atoms inside the solid whereas in the case of hollow cylinder you just have to take into account only the surface phenomena. $\endgroup$– vppCommented Aug 21, 2013 at 6:36
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2 Answers
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A black body is a perfect absorber, and in practice it's difficult to make a material that is a perfect absorber. So to make a black body we choose a material that is as good an absorber as we can find, and form it into a hollow sphere with a small hole in it. The black body is then the hole, not the sphere. It's a nearly perfect absorber because any radiation falling on it is multiply reflected within the sphere. Because the material of the sphere is a good absorber and absorbs most of the radiation at each reflection, the chance that any radiation falling on the opening reflects out again is vanishingly small.
I've never heard of a hollow cylinder being used to model a black body, but the hole at the end of the cylinder would be a black body in a similar way to the hole in the sphere.
answered Aug 20, 2013 at 16:27
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$\begingroup$ I've seen cylinder used as a black body model. Or rather hole on it's side. This was in experimental setup in basic physics laboratory, that was used to check Stefan–Boltzmann law. It was very compact. In fact black body model and detector were both mounted on top of a power supply. Sphere would be quite hard to make, while cylinder was in fact a section of pipe with front faces screwed in. Something that our workshop made without trouble during single workday. $\endgroup$ Commented Nov 20, 2013 at 10:35
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Since a black body is an exact absorber and during practice it's demanding to make things that is really exactly a good absorber so to make black body we choose a material that is as good an absorber as we can obtain it into a hollow Sphere with a small hole in it. The black body is then the hole not the sphere. It is nearly perfect absorber because any radiation falling on it is multiply reflected within the sphere because the material of the sphere is a good absorber and absorbs most of the radiation at each reflection.
