Part II

With metals and conductors the subject complicates. Conductors have a not zero conductivity and a perfect conductor has an infinite conductivity. An object with infinite conductivity or perfect conductor doesn't allow light to enter the object, so 100% of the light is reflected.

With metals the conductivity is very high, but not infinite, unless it is a superconductor, so light is allowed a little to enter the object and what is relected is less of 100%. The light that enters the conductor is very quickly absorbed.

The refraction index now has no sense because there is no refraction, but we can still use Frsnel equations to know how much light is reflected, but in this case the refraction index turns into a complex number because it depend on the dielectric constant and conductivity too.

I our case of making images to make thing easy we can use a fake refraction index, as metal almost reflect all incoming light we can use a big number for refraction index. The value of this number we can find empirically the one that produce the images which we want.

Until now we have ignored the magnetic permeability, that is very near one in many cases. But we with magnetic materials we cannot ignore it and so, we must forget Snell's law, refraction index and Fresnel equations and use instead the characteristic impendance of the medium and its related equations.

For transparent metals things are even more complicated. Conductors don't allow light to penetrate, but if the layer of metal is thin enough the shielding effect of the conductor decrease and some light is allowed to pass throgh. More thiner the layer more light pass and we have a transparent conductor.

The conductivity also depend on the wavelenght, the other two parameters also do. A very thin layer on our microwave window allows us to see inside the microwave owen, but this same thin metallic layer is opaque to microwave wavelenght reflecting all the microwaves and don't allowing to reach us.

Stupidity also evolves!