Mathematical descriptions of opacity

[alikazemi7]

Hi
https://en.wikipedia.org/wiki/Mathematical_descriptions_of_opacity
it is mentioned above that "in non attenuating media, the refractive index and angular wavenumber are related by:

A complex refractive index can therefore be defined in terms of the complex angular wavenumber defined above:

where n is the refractive index of the medium."
Is it right to say that some mediums are attenuating and some non attenuating? and we use

for non attenuating mediums and

for attenuating mediums?
or non attenuating situations is just an approximation and all media are attenuating?

 

[soarce]

All media are attenuating and non attenuating media are just an approximation.
The refractive index is frequency dependent and a media may strongly attenuate in a frequency range and slightly attenuate in other range. Moreover, the (complex) refractive index may depend in general on all parameters of the beam: the intensity, polarization or propagation direction.