ok.... I dunno what absorption resonances are - at least I'm not sure what it is... but X-Rays are absolutely unimportant for this......
Under circumstances that's not the problem anyway... as usually you measure the IoR from transparent materials.
Then there are the two formulas:
c1/c2 = n1/n2
and further: c1/c = n1
and the second one:
tan(a)=c1.............
(at least if I've got that in mind correctly...)
tan(a)/c=n1
(I dunno if that's exactly correct but at least there is a formula where tan is used.....)
With that formula you could also calculate opaque-stuff-IoR.... Which might under circumstances lead to lower IoR.....
