Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: PAOLO137 on 30/03/2015 17:31:45
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When Max Plank and others were trying to find the right equation relating the temperature of a black body and the frequencies of the electromagnetic radiation inside, how did they manage to measure the frequencies with the experimental means available at that time?
Edit: Changed the title to a question.
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As early as 1859, Gustav Kirchoff (http://en.wikipedia.org/wiki/Black-body_radiation#Gustav_Kirchhoff) was using the spectrum of light to look at the wavelengths emitted by hot objects.
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Double slit diffraction was one method available to them. I don't know the extent to which it was used, but they had the math worked out so they could calculate wavelengths with fairly high precision. Interferometers were also important tools.
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When Max Plank and others were trying to find the right equation relating the temperature of a black body and the frequencies of the electromagnetic radiation inside, how did they manage to measure the frequencies with the experimental means available at that time?
Edit: Changed the title to a question.
That depends on the wavelengths which they were trying to observe. Different wavelengths require different methods of observation.
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To be fair, they probably didn't measure the frequency, they calculated it from the wavelength.
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If you look up "ultraviolet catastrophe" in Wikipedia you will see that Planck's equation was in fact a purely theoretical response to a logical absurdity in classical statistical mechanics. It turned out that the stroke of genius in invoking discrete quanta to dispel the anomaly, actually explained the observed photoelectric effect.
Thus the black body spectrum did not require precise experimental determination - the photoelectric effect is a lot easier to measure accurately, and can be explored up to x-ray energies, where the tail of the black body spectrum turns out to be exactly as predicted.
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It turned out that the stroke of genius in invoking discrete quanta to dispel the anomaly, actually explained the observed photoelectric effect.
This seems to be a common misconception. It wasn't Planck himself that quantized radiation. That hypothesis is due to Einstein. What Planck did was to quantize the wall oscillators that make up the black body so that the energy was quantized. That didn't mean that radiation was quantized since theoretically other sources could radiate energy which wasn't quantized in that fashion. This is a very subtle point.
See: http://en.wikipedia.org/wiki/Black-body_radiation
Planck had to assume that the energy of the oscillators in the cavity was quantized, i.e., it existed in integer multiples of some quantity. Einstein built on this idea and proposed the quantization of electromagnetic radiation itself in 1905 to explain the photoelectric effect.
See also: http://galileo.phys.virginia.edu/classes/252/PlanckStory.pdf
Incidentally, it didn’t occur to him that the radiation itself might be in quanta: he saw this quantization purely as a property of the wall oscillators.
This seems to be getting a tad off topic.
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I think Planck was really brilliant, where he might have hesitated was, very tentatively, in the way he wanted to fit his mathematics, and ideas, with the then expected definitions of science, Planck scale is one of the absolutely coolest ideas Ive read about, and if right worth at least worth a Nobel Prize, maybe more :) But he didn't get one from that idea, just as Einsteins relativity never got a Nobel Prize either.
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Many thanks to all the people that have given an answer. I am satisfied to know that at that time there were enough
technical methods to perform such measurement. Paolo.
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Many thanks to all the people that have given an answer. I am satisfied to know that at that time there were enough
technical methods to perform such measurement. Paolo.
I'm curious. May I ask you what led you to question whether it was true or not?