Light propagating in gas scatters off density fluctuations. Most of the scattering proceeds without changing the frequency of light. However, part of the scattered light does change the frequency. This appears as a spectral doublet around the main frequency, called the Brillouin-Mandelstam doublet.The density fluctuations are proportional to volume fluctuations = N V , so the intensity of scattered light I D (V )2 E