Mathematical Descriptions of Opacity - Complex Refractive Index, Extinction Coefficient

Complex Refractive Index, Extinction Coefficient

Recall that in nonattenuating media, the refractive index and wavenumber are related by:

A complex refractive index can therefore be defined in terms of the complex wavenumber defined above:

.

In other words, the wave is required to satisfy

.

Comparing to the preceding section, we have

, and .

The real part of is often (ambiguously) called simply the refractive index. The imaginary part is called the extinction coefficient.

In accordance with the ambiguity noted above, some authors use the complex conjugate definition, where the (still positive) extinction coefficient is minus the imaginary part of .