  We analyze resonant light scattering by an atomic cloud in a regime where
near-field interactions between scatterers cannot be neglected. We first use a
microscopic approach and calculate numerically the eigenmodes of the cloud for
many different realizations. It is found that there always exists a small
number of polaritonic modes that are spatially coherent and superradiant. We
show that scattering is always dominated by these modes. We then use a
macroscopic approach by introducing an effective permittivity so that the
atomic cloud is equivalent to a dielectric particle. We show that there is a
one-to-one correspondence between the microscopic polaritonic modes and the
modes of a homogeneous particle with an effective permittivity.
