Extinction and the optical theorem. Part II. Multiple particles.

The extinction due to a group of identical particles in a fixed orientation is studied. An analytical description based on the Maxwell volume integral equation is presented, describing the interference energy flow due to a group of noninteracting identical particles. As compared to a single particle, the effect of multiple particles is shown to narrow the angular region around the forward direction over which the dominant contribution to the group's extinction cross section occurs. The extinction due to a group of fully interacting nonspherical particles is studied using the discrete dipole approximation. Interparticle interactions are found not to change the essential character of the interference-based energy flow mechanism that describes extinction.