The Mie theory is known to be the first approach to the electromagnetic scattering from homogeneous spheres endowed with all the accuracy of the Maxwell electromagnetic theory. It applies to spheres of arbitrary radius and refractive index and marks, therefore, noticeable progress over the approx imate approach of Rayleigh, which applies to particles much smaller than the wavelength. As a consequence, after the publication of the Mie theory in 1908, several scattering objects, even when their shape was known to be nonspherical, were described in terms of equivalent spherical scatterers. It soon became evident, however, that the morphological details of the actual particles were often too important to be neglected, especially in some wave length ranges. On the other hand, setting aside some particular cases in which the predictions of the Mie theory were acceptable, no viable alternative for the description of scattering from particles of arbitrary shape was at hand. This situation lasted, with no substantial changes, until about 25 years ago, when the exact solution to the problem of dependent scattering from aggregates of spheres was devised. This solution is a real improvement over the Mie theory because several processes that occur, e. g. , in the atmospheric aerosols and in the interstellar medium, can be interpreted in terms of clustering of otherwise spherical scatterers. Moreover, nonspherical particles may be so distributed (both in size and orientation) as to smooth out the individual scattering properties.
