With n2=εm represents the refractive index of the medium. This
calculation method taking account of the fill factor f can thus well
correlate the observed trend. The Urbach's law can describe the
absorption due to glass but the addition of metallic ions can act on
it, as a consequence it is better to consider an approximation law.
The difficulty is not in the approximation of the experimental by a
model but in the good estimate of the nature of the deviations [21].
The radius R, the fill factor f of the NPs and the knowledge of the
dielectric functions of the metal and glass are basic points. Geometry
of NPs (core-shell consideration, shape factor) and optical middles
(lAg, lCu and glass thickness) are necessary to the building of a
complete model.