Résumé
An introduction to the linear response theory will allow us to define the concepts of susceptibility, fluctuation spectra, autocorrelation function... and to connect these quantities through the fluctuation-dissipation theorem. We will then determine the origin of the scattering of light and calculate the Raman and Brillouin spectra within a (classical) macroscopic theory [1]. These calculations will reveal selection rules that will be applied to few examples taken from crystalline solids, liquids, and glasses [2].