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Extinction Coefficient




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.

:: ilde{n}=n-ik

Here, ''n'' is also called the '' Index Of Refraction '', which sometimes leads to confusion, and ''k'' is the ''extinction coefficient''. Both depend on wavelength. In many transparent Dielectric materials, such as glass, the absorption of Visible Light is very small, and ''k'' is then taken to be zero for many applications.

The extinction coefficient ''k'' is related to the Absorption Coefficient \alpha. In a medium that is absorbing light, the decrease in intensity I per unit length z is proportional to the value of I. In mathematical form this is

:: rac{dI\left( z ight)}{dz}={-\alpha I\left(z ight)}
which has the solution

::{I\left(z ight)}={I_0 e^{-\alpha z}} ,

:where I_0 is the intensity of the electromagnetic radiation at the surface of the absorbing medium.
The extinction coefficient ''k'' is given by

::k={ rac{\lambda}{4\pi}}\alpha ,

:where λ is the vacuum Wavelength .