Dielectric constants and refractive indices of the matter are generally complex quantities. Considering an electromagnetic wave entering a homogeneous medium of complex refractive index n=m+ik, it is possible to demonstrate that its intensity progressively decays depending on its wavelength λ and on the complex part k of the refractive index of the considered medium. Transparent medium correspond to medium having k=0 (i.e. real refractive index).
For instance, considering the amplitude of the electric field E(0) entering the medium, its value after traveling in it for a distance z will be reduced at E(z)=E(0)exp[ -ωkz/c] being ω the wave pulsation and c the light speed constant.