Polarization Of Thermal Radiation In A Symmetric Cavity

Formulas are derived for expressing the state of polarization of thermal radiation in an axially symmetric cavity. These formulas are then used to demonstrate that thermal emission from the cavity is unpolarized when the cavity is in thermodynamic equilibrium and that any state of polarization may exist in a diathermanous medium which is enclosed by perfectly reflecting walls. It is also demonstrated that formulas which prescribe the emittance or reflectance of the cavity, neglecting polarization effects, are inherently erroneous.