Light Absorption and Linear Dichroism

1 Extinction Index, Absorption Coefficient, Optical Density 

An electromagnetic wave propagating with velocity v in a medium is described by the wave equation  

Assuming E(r, f) = E(r)exp(—icot) we exclude the time dependence and get the Helmholtz equation  

For the plane wave E(r) oc expikr and we obtain the dispersion relation = Epco /c. For the absorbing medium the wavevector amplitude k becomes complex  

The absorbing medium can be described in terms of a complex refraction index n = n + iK where n is real refraction index and k is real extinction index. For nonmagnetic medium p 1 and (n + iK) = rP — h + linK = e = e + e we find the relations between the components of e and n  

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