Extinction and the optical theorem. Part I. Single particles.

We study the extinction caused by a single particle and present a conceptual phase-based explanation for the related optical theorem. Simulations of the energy flow caused by a particle's presence in a collimated beam of light demonstrate how the extinction process occurs. It is shown that extinction does not necessarily cause a reduction of the energy flow along the exact forward direction. Implications regarding the measurement of the single-particle extinction cross section are discussed. This work is extended to noninteracting and interacting multiparticle groups in Part II [J. Opt. Soc. Am. A25, pp. 1514 (2008)].