Light scattering by anisotropic spherical particles: Rayleigh-Gans approximation versus T-matrix theory

We calculated the scattering of light by composite anisotropic particles embedded in an isotropic matrix by using the T-matrix ansatz. The model scatterer is assumed to consist of a small central isotropic particle coated by a much larger region in which the optical tensor is radially anisotropic. We determine the region of validity of Rayleigh-Gans approximation (RGA) by comparing approximate values of the scattering cross-section and the dispersion index with the results computed from the exact solution. Both of the scatterers with isotropic and radially anisotropic annular layers have been studied. Our results show that RGA for anisotropic scatterers gives less accurate estimates than when the scatterer is isotropic. In the small particle limit, we have calculated the value of dielectric contrast of effective isotropic scatterer as a function of anisotropy parameter and the layer thickness. We have found that averaging the dielectric tensor over angles gives correct results only if the anisotropy parameter is small.