Using the classical Mie scattering theory, we compute the energy density of an arbitrary partial wave （e.g., the nth order） and then determine that the interaction between an incident planar wave and a sphere of radius a is the one between the sphere and those partial waves the order of which satisfies n ≤ ka. We also provide a simple expression to describe the diffracted wave in which the angle-dependent functions are employed. The difference between the accurate and the approximate expressions is demonstrated by numerical calculation.
