Absorption spectrum of neighboring dielectric grains

It is shown that the dipole approximation is inadequate for describing the absorption properties of dielectric powders. The case of a pair of spheres modeling two neighboring grains is solved in a complete multipolar treatment. While the dipole approximation yields one resonance at a frequency in disagreement with experiment, an infinite set of solutions is now found covering in particular the experimental value and predicting the presence of other resonance peaks.