AbstractIn a nematic liquid crystal, there exist thermally excited fluctuations in the orientation of the mean alignment direction of the molecules (represented by a unit vector, called the director). These fluctuations modulate the dielectric tensor Ê of the medium; as a result there is an intense and strongly depolarized scattering of light. It is shown that these orientation fluctuations can be decomposed into two independent modes; following the notation of Frank [1], the types of distortion are splay and bend for one mode, splay and twist for the other. The contribution of each mode to the intensity of the scattered light may be calculated as a function of temperature and scattering wave vector [2]; the results are in good agreement with the measurements of Chatelain [3] and permit, in principle, a determination of the three elastic constants kii of the liquid crystal.The time dependence of these fluctuations. on the other hand, is derived from the hydrodynamic equations of Ericksen and Leslie [4] which describe the coupling between the rotations of the director and the overall motions of the fluid. This coupling can be described using five independent coefficients αi, which are equivalent to viscosities. One finds that in usual systems the two fluctuation modes do not propagate, but are purely dissipative with a characteristic relaxation time. This time is a function of the wave vector, the elastic coefficients k and the viscosity coefficients αi.We have verified the predictions of the theoretical model by a study of the spectrum of the scattered light, using a high resolution laser light beating spectrometer. The observed spectrum consists of a Lorentzian line centered about the incident frequencies (a Rayleigh line), which confirms the purely dissipative nature of the orientation fluctuations. Measurement of the line width permits a determination of the relaxation time and its dependence upon wave vector. By comparing our experimental results with the analytic formulae of the theory we have been able to deduce the anisotropic viscosity coefficients of the nematic phase [5]; a determination which is difficult, if not impossible, by conventional hydrodynamic methods.