No one seems to be interested so I present my own explanation to see if some one agrees or not. consider the case of semiconductor transmission, since there is a band gap (see schematic fig, energy values increase towards right and transmission upwards) so material is fairly opaque in highe energy range but there is abrupt change at the onset of absorption and in IR range it becomes transmitting. Transmission stays flat there because there are only interband transitions. On the other hand for metal, we almost similar behavior by the metal in the high energy range and at the band gap (band gap does not necessarily mean a material is semiconducting until and unless we have several other calculations to support SC state) but in low frequency region we again fall in the transmission. Possible explanation for this fall might that electrons very close to surface absorb EM waves, vibrate and loose energy. This energy can be seen as reflection (which will be less than transmission but still can be significant). Finally not to mention that there is a reasonably good amount of absorption as well. What does this explanation sound like?
Secondly let us say we have reflectance curve instead of transmission then how can I calculate n, k vaules from it? I know the reflectivity formula and I am afraid to use (alpha= 4pi*k/lambda) to find k values. It gives wron answer.
