The dependence of the far-field coherence and radiant intensity of light scattered from a liquid crystal on the correlation length for the fluctuations of molecules in the crystal are analyzed theoretically. The results are useful for understanding data obtained in scattering experiments that study the hydrodynamic properties of liquid crystals excited into internal flow by a dc electric field or by application of sound vibrations. It is found that if the molecular correlation length is small relative to the dimensions of the illuminated volume, the scattered field behaves according to the van Cittert–Zernike theorem, and the motion of the molecules is better studied by measurement of the radiant intensity. If instead the molecular correlation length is much longer than the dimensions of the illuminated volume, the van Cittert–Zernike theorem does not apply, and the spatial correlation properties of the motion of the molecules can be studied by measurement of the far-field coherence.
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