  Light scattering due to the interaction of photons and acoustic waves present
in a dilute inert gas is analyzed through the use of irreversible
thermodynamics. The dispersion relation, which governs the dynamics of the
density fluctuation of the gas allows the establishment of a simple criterion
for the corresponding Rayleigh-Brillouin spectrum to be observed. The criterion
here proposed allows a clear physical interpretation and suggests
generalizations for other interesting physical scenarios.
