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In this paper we describe the propagation of the coherent component of an electromagnetic wave in a colloidal system with large inclusions using an effective-medium approach. We show that the effective medium is nonlocal (spatially dispersive) and derive expressions for the nonlocal longitudinal and transverse components of the dielectric response, ${ϵ}^L$ and ${ϵ}^T$. Numerical calculations of the wave-vector dependence of these response functions are displayed. The dispersion relation for the transverse modes is calculated and compared with the results obtained using well-known approximations for the effective index of refraction. It is also shown that some of these approximations have actually a nonlocal nature, explaining why it is not possible to use them to calculate, for example, the reflection properties of the colloidal system using conventional continuum electrodynamics. We also calculate the effective nonlocal electric permittivity $ϵ$ and effective nonlocal magnetic susceptibility $\mu$ and show that this more traditional description is equivalent to the one using ${ϵ}^L$ and ${ϵ}^T$.

Using a multiple-scattering formalism, we derive closed-form expressions for the coherent reflection and transmission coefficients of monochromatic electromagnetic plane waves incident upon a two-dimensional array of randomly located spherical particles. The calculation is performed within the quasi-crystalline approximation, and the statistical correlation among the particles is assumed to be given simply by a correlation hole. In the resulting model, the size of the spheres and the angle of incidence are both unrestricted. The final formulas are relatively simple, making the model suitable for a straightforward interpretation of optical-sensing measurements.

We compare a recently developed coherent-scattering model for the reflectance of light from a turbid colloidal suspension of particles with experimental measurements. The experimental data were obtained in an internal reflection configuration around the critical angle using a glass prism in contact with a monodisperse colloidal suspension of latex particles, and a polydisperse suspension of TiO2 particles. First, we review the coherent scattering model and extend it to the case of polydisperse suspensions in an internal reflection configuration. The experimental data is then compared with results of the coherent scattering model and results obtained assuming that the colloidal system can be treated as a homogeneous medium with an effective index of refraction. We find that the experimental results are not compatible with the effective medium model. On the other hand, good fits to the experimental curves can be obtained with the coherent scattering model.