In Section 2.2 we discussed the nature of collisions between electrons and ions, suggesting that we might view plasma as two separate fluids, an electron fluid and an ion fluid, as the electrons and ions have substantial mass differences. Yet the two fluids are not independent since their constituents undergo collisions and thus exchange momentum and energy. There is, however, a third fluid to be considered, namely radiation. In Chapter 3 we found that we could describe radiation as a propagating electromagnetic wave. Since the radiation, though, undergoes interactions with the electron fluid involving exchanges of momentum and energy, the properties of radiation may also be treated with methods not much different from those for particles. In treating the radiation as a fluid of point, massless particles – which are called photons – we can derive an equilibrium distribution function for radiation in much the same way we did in Chapter 2 for ions and electrons.

Radiation as a fluid and the Planck distribution function

Let us begin our discussion of radiation transport by treating the radiation as a photon fluid. Each of these photons carries an energy E with associated frequency ν, where E = hν and ν = ω/2π. These massless particles – the photons – also carry momentum p = E/c = hν/c. In the absence of any matter the photons travel in straight lines at the speed of light, c. Hence, they can transport energy and momentum long distances, very rapidly. Even in the presence of matter, the mean free path of the photons may be much longer than the collisional mean free path of the particles with mass; the speed of photons is much greater than thermal velocities of the massive particles. Radiation also exhibits pressure, and is thus capable of doing work. As we shall see, the interaction of radiation with matter has a special importance. Since the radiation energy is propagated at the speed of light, relativistic effects must be considered, although they are important only in certain respects.