We examine the scattering properties of particles contained in absorbing media. Rather than consider energy fluxes through arbitrary integrating spheres, we examine the extinction from its fundamental definition: the energy removed from the plane wave, or incident beam. The resulting energy received by a detector contains two terms: one the result of the incident beam traversing through the medium that would have occurred if the particle were not present, and a correction term due to the presence of the particle. Both terms have the same dependence on the pathlength that the beam travels between two arbitrarily located parallel planes and are independent of where the particle is located within the medium. The result is that the definition of the extinction cross section is not dependent on a reference plane or the particle location within the medium.
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