  Despite the quantum nature of the process, collective scattering by dense
cold samples of two-level atoms can be interpreted classically describing the
sample as a macroscopic object with a complex refractive index. We demonstrate
that resonances in Mie theory can be easily observable in the cooperative
scattering by tuning the frequency of the incident laser field or the atomic
number. The solution of the scattering problem is obtained for spherical atomic
clouds who have the parabolic density characteristic of BECs, and the
cooperative radiation pressure force calculated exhibits resonances in the
cloud displacement for dense clouds. At odds from uniform clouds which show a
complex structure including narrow peaks, these densities show resonances, yet
only under the form of quite regular and contrasted oscillations.
