Abstract
The extinction cross section for an arbitrary spherical particle that is embedded in an absorbing medium is derived by considering that which is measured in an actual extinction experiment. The resultant “optical theorem” is similar in form to the optical theorem for particles in a nonabsorbing medium. Calculations of extinction by germanium particles that are embedded in a polyethylene medium at room temperature have been performed for far-infrared frequencies (50–200 cm −1). If the imaginary part of the medium refractive index is ignored, then particulate extinction is underestimated by factors of between 2 and 4 over the frequency range of interest for 0.02-μm-radius spheres. In addition, there is a significant shift of a bulk germanium absorption maximum, which is not predicted when the medium is assumed to be nonabsorbing. For larger particles, the relative error decreases but still may be significant.
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