The interaction of light with scattering matrixes, particularly particles suspended in a liquid, is described as it relates to the use of a technique known as grating light reflection spectroscopy (GLRS). Analysis of the singular behavior of light reflected from a transmission diffraction grating in contact with a scattering matrix has shown that the effective dielectric function of the medium in contact with the grating can be measured using a reflection method. The relative contributions of the real and imaginary components of the effective dielectric function are probed as mean particle radius increases, shifting the forward scattering regime from the Rayleigh limit to the Fraunhofer diffraction approximation. It is shown that GLRS provides substantial information regarding the characteristics of colloids and suspensions. Further investigations could, in theory, yield particle shape and density distribution information as well.