  We compare the behavior of propagating and evanescent light waves in
absorbing media with that of electrons in the presence of inelastic scattering.
The imaginary part of the dielectric constant results primarily in an
exponential decay of a propagating wave, but a phase shift for an evanescent
wave. We then describe how the scattering of quantum particles out of a
particular coherent channel can be modeled by introducing an imaginary part to
the potential in analogy with the optical case. The imaginary part of the
potential causes additional scattering which can dominate and actually prevent
absorption of the wave for large enough values of the imaginary part. We also
discuss the problem of maximizing the absorption of a wave and point out that
the existence of a bound state greatly aids absorption. We illustrate this
point by considering the absorption of light at the surface of a metal.
