Landau–Placzek ratio is a ratio of the integrated intensity of Rayleigh scattering to the combined integrated intensity of Brillouin scattering of a triplet frequency spectrum of light scattered by homogenous liquids or gases. The triplet consists of two frequency shifted Brillouin scattering and a central unshifted Rayleigh scattering line split. The triplet structure was explained by Lev Landau and George Placzek in 1934 in a short publication, summarizing major results of their analysis. Landau and Placzek noted in their short paper that a more detailed discussion will be published later although that paper does not seem to have been published. However, a detailed discussion is provided in Lev Landau and Evgeny Lifshitz's book.
The Landau–Placzek ratio is defined as
where 
* is the integral intensity of central Rayleigh peak 
* is the integral intensity of Brillouin peak.
The Landau–Placzek formula provides an approximate theoretical prediction for the Landau–Placzek ratio,
where 
* is the specific heat at constant pressure 
* is the specific heat at constant volume.