We use a nonintrusive optical technique for heterodyne detection of the light scattered elastically by the molecules of a moving transparent gas, a phenomenon known as Rayleigh scattering. It can be shown that the signal that comes out of the photodetector is proportional to the spatial Fourier transform as a function of time of the density fluctuations, for a wave vector given by the optical set-up. This is the only technique we are aware of that can study density fluctuations inside a flow. In this paper we present results obtained from a supersonic axisymmetric air jet. The signal that comes out of the photodetector is processed, and the power spectrum calculated. In the spectrum, density
fluctuations of two different origins can be identified: acoustic, that is, those that propagate at the speed of sound and are related to pressure variations, and entropic, those that have constant pressure and are convected by the flow. At certain locations we have found an additional peak related to the interaction between the flow and the shock structure. Furthermore, Rayleigh scattering can be used to visualize the shock structure of the flow. We provide supporting images for our results.