Thermodynamics and Energy


Kirchhoff ’s Law of Thermal Emission: Blackbody and Cavity Radiation Reconsidered

Authors: Pierre-Marie Robitaille

Kirchhoff’s law of thermal emission asserts that, given sufficient dimensions to neglect diffraction, the radiation contained within arbitrary cavities must always be black, or normal, dependent only upon the frequency of observation and the temperature, while independent of the nature of the walls. In this regard, it is readily apparent that all cavities appear black at room temperature within the laboratory. However, two different causes are responsible: 1) cavities made from nearly ideal emitters self-generate the appropriate radiation, while 2) cavities made from nearly ideal reflectors are filled with radiation contained in their surroundings, completely independent of their own temperature. Unlike Kirchhoff’s claims, it can be demonstrated that the radiation contained within a cavity is absolutely dependent on the nature of its walls. Real blackbodies can do work, converting any incoming radiation or heat to an emission profile corresponding to the Planckian spectrum associated with the temperature of their walls. Conversely, rigid cavities made from perfect reflectors cannot do work. The radiation they contain will not be black but, rather, will reflect any radiation which was previously incident from the surroundings in a manner independent of the temperature of their walls.

Comments: 11 Pages. An earlier partial version of this work was archived on as follows: Robitaille P.-M. and Robitaille J. L. Kirchhoff ’s Law of Thermal Emission: What Happens When a Law of Physics Fails an Experimental Test? viXra:1708.0053.

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[v1] 2018-06-08 04:52:51

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