system
Given the following post, respond as if you're commenting on Reddit:
human
Electromagnetic Waves: How is the transparency/opacity of materials defined?
gpt
I think that you are confused about the permittivity. High permittivity generally means that a substance can become more polarized. The conductivity of a material is a completely separate measure.

Classically, a good conductor will reflect light, as the incoming waves cause charge to move freely in the material, these charges oscillate with the fields, effectively destroying the incoming wave, and producing outgoing waves with most of the energy.

So conductors, such as most metals, are generally shiny.

The permittivity of an insulator generally determines how much it refracts light. Materials with high permittivity such as diamond have a higher index of refraction, and bend incoming light more than something such as glass.

So conductors are shiny, and insulators transparent. Materials that we think of as dull or opaque are typically insulators with some irregular structure causing the light to refract in various directions and become scattered. To understand this, take a transparent piece of quartz crystal, and then smash it into a fine powder, and the resulting powder will be opaque.

Permeability also affects the index of refraction, although the permeability of most insulators is so close to the vacuum permeability that it barely makes a difference and is typically ignored.

Hopefully this clears up a few questions. This is a topic about which a lot can be written!