RETARDATION CORRECTION TO THE LORENTZ-LORENZ FORMULA FOR THE REFRACTIVE-INDEX OF A DISORDERED SYSTEM OF POLARIZABLE POINT DIPOLES

An approximate expression for the effective dielectric tensor of a flu id or suspension of spherical particles with a polarizable point dipol e at the center is derived by a selection of terms in the cluster expa nsion. Full account is taken of the effect of retardation. The effecti ve dielectric tensor depends on wave vector and frequency. In the limi t of zero frequency the derived expression reduces to the wave-vector- dependent tensor found from electrostatics by the same procedure. In t he limit of zero frequency and zero wave vector it reduces to the Clau sius-Mossotti formula. The transverse part of the tensor may be regard ed as a generalization of the well-known Lorentz-Lorenz formula for th e refractive index. It is shown that the generalization leads to unphy sical results if the damping of the individual spherical particles is too small. This implies that correlation corrections must be taken int o account.