The parameter used to describe the interaction of electromagnetic radiation with matter is the complex index of refraction, ñ, which is a combination of a real part and an imaginary part.

Here, n is also called 'index of refraction' which sometime leads to confusion, and k is called the 'extinction coefficient'. In a dielectric material such as glass, none of the light is absorbed and therefore k = 0.

Before a more detail definition of the extinction coefficient k is considered, let us first look at the absorption coefficient α. In a medium absorbing light, the decrease in intensiry I per unit lenght z is proportional to the value of I. In mathematical form this means:

where α is the absorption coefficient.

This can be integrated as:

where I₀ is the value of the intensity at the surface of the absorbing medium.

The extinction coefficient k is then defined as

where λ is the vacuum wavelength