Complex Refractive Index and Absorption

When light passes through a medium, some of the light is absorbed. This can be conveniently taken into account by defining a complex index of refraction  

the real part of the refractive index n indicates the phase velocity, while the imaginary part k indicates the amount of absorption loss when the electromagnetic wave propagates through the medium material. Both n and k are relative to the frequency. When k 0, light is absorbed. When k = 0, light travels infinitely without loss. An absorption spectrum of a material can be used to determine its full complex refractive index as a function of wavelength. 

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