Many-particle systems: Models of inhomogeneous matter

Inhomogeneous matter of porphyric structure may be modelled by composite systems of many spherical particles embedded in a homogeneous matrix. These systems are simple enough to enable the microscopic interpretation of macroscopic physical properties. In the present paper a review is given of detailed models for the optical properties beyond the effective medium theories, which include direct electromagnetic interactions among the particles of such systems. In special, the elementary optical excitations prove to be highly sensitive to the microstructure of the samples. They allow to analyze aggregation processes and to distinguish between coagulation and coalescence aggregates, the latter being responsible for the electric percolation.