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Kirchhoff's Law: Author's Comments

 

There was a fundamental mistake…namely, the paying of attention solely to the result given by experimental measurements and entirely ignoring the inner realities of the inner processes.

 

Max Planck, The New Science, 1959.

 

 

 

Back to Kirchhoff


Kirchhoff's Law of Thermal Radiation was formulated in 1859.  It is at the same time the simplest and least understood law in physics today.  Kirchhoff's law was describes how an object radiates when placed in thermal equilibrium within the confines of an enclosure.

Kirchhoff considered two types of enclosures, isothermal and adiabatic.  An isothermal enclosure was typically made from graphite and had perfectly absorbing walls.  On the other hand, an adiabatic enclosure was made from highly polished metals and had perfectly reflecting walls.  Kirchhoff believed that provided an object was within the confines of either enclosure, that the radiation emitted (or thermal spectrum) was characterized solely by the temperature of the resulting system (object and enclosure).  Thermal radiation became independent of the nature of the emitting object.

Kirchhoff's Law is central to the development of thermodynamics. Unfortunately, many believe that a temperature can be obtained from a thermal spectrum without knowledge of whether or not the object of interest was in fact in thermal equilibrium with an enclosure. This can lead to serious errors in that the temperatures obtained can be completely invalid.

Recently, the experimental justification for Kirchoff's Law has been re-evaluted.  It has been discovered that Kirchhoff improperly overextended his law to include the perfect reflector (an adiabatic enclosure).  In actuality, Kirchhoff's Law is valid only for the perfect absorber.  The consequences are serious.  As a result, blackbody radiation becomes critically dependent on the nature of the object in question.  A full treatment is provided in the December 2003 issue of "IEEE Transactions on Plasma Science" by Dr. Robitaille.  This is a special issue dedicated to Space and the Cosmos.  An APS abstract has also been presented on this topic as found above.