Formulas for the spectrum of light scattered by the fluctuations of density and temperature, or pressure and entropy, in a thermally conducting viscous fluid are derived by applying the fluctuation‐dissipation theorem directly to the scattering configuration. The language of simple circuits and familiar logic of Nyquist are used, and particular attention is paid to the phasing between the fluctuations of different variables. Spectral profiles and the correlation between the fluctuations of the variables indicated by these rather complicated formulas are also discussed in terms of physical concepts (normal modes, degrees of freedom, and the relaxation due to thermal conduction). Contact is made in this way with both classical and recent theories of light scattering. Limitations in these other theories due to inadequate phasing of fluctuations are described and extensions are proposed. It is established that different sets of approximate normal coordinates are required for an adequate simple description of spectra, depending on the ratio of the frequencies of relaxation and acoustic resonance. It is also shown that the traditional neglect of the contributions of temperature fluctuations may lead to serious error in the analysis of Brillouin spectra of scattered light. The extension of the new theory to fluids with relaxing heat capacity and compressibility or a different structure factor is sketched briefly.