My current understanding is that materials found in nature attenuate light as a function of some fixed constant, according to this equation: $ I(x) = I_0 e^{-\kappa_v \rho x} $, where $ I_0 $ is the initial intensity, x is distance inside the material, $ \kappa_v $ is 'opacity', and $ \rho$ is mass density. I am assuming that most materials that I interact with on a day to day basis have some value of $ \kappa_v $ that is between $0$ and infinity. This means if I put an instrument that measured light in a hollow iron sphere which had a wall thickness of say a meter, and put the sphere in orbit around the sun, that a sensitive enough light detector would still be able to pick up some light penetrating the sphere.
Is this correct, or is that equation just some hand-wavey engineering approximation? Are there any materials for which $\kappa_v$ is zero or infinite? I.e. where absolutely no em radiation penetrates whatsoever (maybe a superconductor?) or passes light with zero attenuation of intensity?
