Optical Characterization of a Turbid Colloid by Light Reflection around the Critical Angle

Abstract

We present a scattering theory model for the coherent reflectance of light from a turbid colloid in an internal reflection configuration. We compare with experimental measurements obtained with turbid suspensions of spherical latex particles and irregularly shaped TiO2 particles. When the colloidal particles are not small compared to the wavelength of radiation, the reflectance curve as a function of the angle of incidence predicted by the scattering theory model, differs appreciably from that predicted by Fresnel’s relations with an effective refractive index. Therefore, when the particles size is comparable or larger than the wavelength, one can not determine the effective refractive index in the usual way using a critical angle refractometer. We provide some insight to why Fresnel’s relations do not work with turbid media.