Why are insulators transparent to visual light and metals are not?

Answer 1

In 1909, Ernest Rutherford fired alpha particles (helium nuclei) at a very thin sheet

of gold foil. Many of the alpha particles passed through but some were deflected

at an angle and some surprisingly bounced back. This experiment showed that the

atom had a dense but very small positive core, the electrons were far away from

the very small positive core (nucleus), and the electrons were separated from the

nucleus by a lot of empty space. The atom is more than 99.999999% empty space!

Like clear glass insulators, it seems that everything should be transparent to visual

light. However, metals have a loose sea of electrons that makes visible light bounce

back and the metal appears shiny. For opaque objects like metals, most of the light

is either reflected by the object or absorbed and converted to heat. For transparent

objects like air and clear glass, the light travels through it.

Another contributor passes through and pauses to observe:

The question is perplexing. It starts off by asserting glibly that insulators ...

which in my experience includes wood, rubber, and concrete ... are transparent

to visual light.

Seems to me that if insulators were transparent to visual light, then we would

always be able to see every conducting wire inside a cable of many wires, since

each and every one is covered in an insulating jacket.