Mathematical descriptions of opacity

When an electromagnetic wave travels through a medium in which it gets attenuated (this is called an "opaque" or "attenuating" medium), it undergoes exponential decay as described by the Beer–Lambert law. However, there are many possible ways to characterize the wave and how quickly it is attenuated. This article describes the mathematical relationships among:

• attenuation coefficient;
• penetration depth and skin depth;
• complex angular wavenumber and propagation constant;
• complex refractive index;
• complex electric permittivity;
• AC conductivity (susceptance).

Note that in many of these cases there are multiple, conflicting definitions and conventions in common use. This article is not necessarily comprehensive or universal.