[labview1958 (OP)]

Why is glass transparent and metallic copper is opaque?



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[Soul Surfer]

The most important feature about copper that makes it opaque is that it is a conductor of electricity.  This is because the atoms in the solid are bonded together in such a way to allow the outer electrons freedom of movement through the material.  Because it is a conductor it effectively "shorts out" the electrostatic part of electromagnetic waves (light)and this reflects the energy from the surface and prevents it from flowing through the body of the copper in a coherent way. 

Now glass is an insulator and electromagnetic energy can enter the material but that's not enough to make it transparent The next requirement is that the material itself does not absorb the frequencies of light that are being used.  Many insulators are coloured because they absorb light of different frequencies.

Finally there is one important additional requirement.  Glass is a uniform material and is often prepared with a smooth surface this ensures that the light is not dispersed through the material and glass is transparent.  Only a very slight disturbance of the surface will make the glass frosted or obscured.

[lyner]

Brill explanation!
Well put.

[yor_on]

Interesting, when I think of why you can see through glass I think about its molecular structure primarily. Its ordered similar to a fluid but rigid, I guess that's why some say it is a very slow running 'fluid' . If you consider water, or a gas the light have ways through it, the molecules don't absorb as much of the viewable spectrum as there are 'holes' between them. Also our viewable light doesn't get absorbed as much by the electronsclouds in glass. But that depends on what type of glass it is.