  We provide a general microscopic theory of the scattering cross-section and
of the refractive index for a system of interacting colloidal particles, exact
at second order in the molecular polarizabilities. In particular: a) we show
that the structural features of the suspension are encoded into the forward
scattered field by multiple scattering effects, whose contribution is essential
for the so-called "optical theorem" to hold in the presence of interactions; b)
we investigate the role of radiation reaction on light extinction; c) we
discuss our results in the framework of effective medium theories, presenting a
general result for the effective refractive index valid, whatever the
structural properties of the suspension, in the limit of particles much larger
than the wavelength; d) by discussing strongly-interacting suspensions, we
unravel subtle anomalous dispersion effects for the suspension refractive
index.
