Message: Re: Imaginary index and Absorption Length  Not Logged In (login) 
OK, I think I found the answer to my question on my own:
If the reflectivity of a surface is not specified via "REFLECTIVITY" and the complex refractive index is given (by "REALRINDEX" AND "IMAGINARYRINDEX"), the surface reflectivity is calculated from the complex refractive index via the Fresnel equations.
Note that this only makes sense for dielectric_metal surfaces, as the reflectivity simulates absorbing dirt for dielectric_dielectric surfaces.
As far as your question is concerned, Nicolas, I think refractive indices have no influence on the absorption length. In Geant4, the complex refractive index is a property of the surface and not of a material. Therefore, no material properties can be derived from it. If you want to specify the absorption length via the complex refractive index, you have to calculate the absorption length spectrum from the energy/wavelength dependent complex refractive index on your own and pass it to the material (via "ABSLENGTH"). I am not an expert, but this is what I found when searching for the answer to my question. Perhaps you could test if I am correct, by defining a single box without absorption length, adding a surface with complex refractive index (large imaginary part) and refractive index (otherwise optical photons may be killed) and generating optical photons in the centre of the box. If the absorption length is influenced by the complex refractive index, the photons should be absorbed directly. In the other case, they should travel through the box. Best regards, Erik

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