This paper concerns the dynamic light scattering by suspensions of polydisperse hard spherical particles. It is concluded that for relatively high volume fractions and fairly narrow distributions, the light scattering correlation function should, to a good degree of approximation, be composed of two independent modes with well-separated decay times. The faster decaying mode describes collective stochastic compression–dilation motions and is present even for a monodisperse system. The slower decaying mode describes the exchange of different species. The relative mode amplitudes are calculated in the Percus–Yevick approximation for hard spheres. The two decay times of fast and slow mode are associated with diffusion coefficients D+ and D−, respectively. In the case of scattering power polydispersity D+ and D− are rigorously identified with collective and self-diffusion, respectively. In the case of size polydispersity D+ and D− can be associated with “average” collective and self-diffusion coefficients. The considerations are limited to zero scattering wave vector and volume fractions above 0.15. Comparison with experimental results from other papers are in favor of the theory.
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