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 Quote by Gokul43201 This is only partially correct. Most metals have a high reflectivity in the visible range and are highly transparent in the high UV range. The reflectivity in the visible range is, as you point out, due to the excitation of surface plasmons (collective excitations of the free electron gas). However, the electron gas, has a natural frequency (the plasma frequency) above which it is not very good at responding to the EM-field of the incident light. For most metals, the plasma frequency (which is a function of the free electron density and the conduction band effective mass) lies in the UV range. Yes, that is essentially correct, but it hardly makes the problem any easier to solve. The vast majority of insulators are opaque, and this is true even of amorphous materials. Glasses are kind of special in that they are essentially supercooled liquids.
its funny...i had a thought....but its more of a question really...

would metals be more reflective because of their layered crystalline structure as compared to insulators which less so?
I say this because each crystal layer within the metal could be acting like a barrier and would aid the reflection rather than the transmission.

It wouldn't be difficult to see that higher eV energy (UV) would penetrate as they fall under the higher exiton polaritons, whose energy levels are considerable for the fields available in the metals for stopping or diverging such photons...