.. note:: This site is currently under construction.

Smuthi returns the extinction cross sections for the reflected and the transmitted wave. For the
scattering of a plane wave by particles in a homogeneous medium, the extinction cross section is usually defined as the
sum of total scattering and absorption cross section.

In Smuthi, we instead use what is usually referred to as the
`optical theorem <https://en.wikipedia.org/wiki/Optical_theorem>`_ to define extinction. That means, the extinction
cross section for reflection (transmission) refers to the destructive interference of the scattered signal with the
specular reflection (transmission) of the initial wave. It thereby includes absorption in the particles, scattering,
and a modified absorption by the layer system, e.g. through incoupling into waveguide modes. If the particles lead to,
say, a higher reflection than the bare layer system without particles, the extinction can also be negative.

.. todo:: further clarify definition of cross sections in the context of planar layer interfaces.