To demonstrate universality of blackbody radiation Kirchhoff used a cavity with walls of graphite into which he put different objects of different materials and temperature, waited until the object reached radiative equilibrium with the graphite walls and observed the radiated spectrum through a peep hole. Kirchhoff then observed radiation intensities only depending on temperature and frequency according to Planck’s Law (with simplified high frequency cut-off):
- for ,
- for ,
where and are certain given constants. From this observation Kirchhoff declared universality of blackbody radiation.
Kirchhoff observed to his disappointment that removing the graphite destroyed universality, which however returned by adding a small amount of graphite. This allowed Kirchhoff to maintain his declaration of universality of blackbody radiation, as a mystification to coming generations of physicists dreaming of universality as the ultimate expression of deep understanding.
If we now analyze Kirchhoff’s universality from the perspective of the previous post, we understand that the graphite serves as the reference blackbody effectively establishing universality. Again the universality expresses nothing but radiative equilibrium using a blackbody of graphite as reference. The mystery of universality of blackbody radiation is thus revealed as the no-mystery of a form of standardization.
For an introduction to classical work with an empty cavity as an abstract (mystical) universal reference blackbody, see An Analysis of Universality in Blackbody Radiation by P.M. Robitaille.