106.

When two ray components $𝖪$ and ${𝖪}^{\prime }$, polarized at right angles to each other, are non-coherent, $𝖪$ and ${𝖪}^{\prime }$ are also the principal intensities, and the entropy radiation is given by [eqn:(141)] (141). The converse proposition, however, does not hold in general, that is to say, the two components of a ray polarized at right angles to each other, which correspond to the principal intensities $𝖪$ and ${𝖪}^{\prime }$, are not necessarily non-coherent, and hence the entropy radiation is not always given by [eqn:(141)] (141).

This is true, e.g., in the case of elliptically polarized light. There the radiations $𝖪$ and ${𝖪}^{\prime }$ are completely coherent and their entropy is equal to $𝖫(𝖪+{𝖪}^{\prime })$. This is caused by the fact that it is possible to give the two ray components an arbitrary displacement of phase in a reversible manner, say by total reflection. Thereby it is possible to change elliptically polarized light to plane polarized light and vice versa.

The entropy of completely or partially coherent rays has been investigated most thoroughly by M. Laue.M. Laue, Annalen d. Phys. 23, p. 1, 1907. For the significance of optical coherence for thermodynamic probability see the next part, [sect:119.] Sec. 119.

[Stationary Field of Radiation] VElectrodynamical Processes in a Stationary Field of Radiation