Star Trek IV: The Voyage Home - transparent aluminum whale tank

Rhetorical contrast — Scotty's fictional "transparent aluminum" used to illustrate that real metals cannot be transparent because of free electrons.

Today I was going to start talking about another structural material, which is sort of the antithesis, at least in terms of toughness, and that's glass. What is the property of glass that makes it so important that we want to use it even though it's brittle? It's transparent. You won't find any metal that's transparent. If you want to know why, go back to your solid state physics. It's because metals have free electrons. By the very nature of metallic bonding you have free electrons, and free electrons can absorb virtually any wavelength of electromagnetic energy, including light. Whereas in glass, there are certain bands — the conduction band and the valence band — and forbidden regions in between, quantum mechanical forbidden regions, that electrons cannot have those energies in the solid. Because of that, you can come up with materials that are transparent to visible light. Metals will never be one of those, in spite of what Scotty said in Star Trek IV: The Voyage Home, where they had transparent aluminum. It wasn't metallic aluminum, because metals are not transparent.

Used to make the point that metals are not transparent because they have free electrons that absorb light. Tom's standing classroom joke on the ionic/covalent vs. metallic bonding distinction.

If you want to be a materials scientist, we can explain why. We can explain why many of the ceramics — the ionic and covalent bonded materials — are transparent. They don't have free electrons. No metals are transparent, except for the one transparent metal we know of. There's no Star Trek fans? Transparent aluminum — that's what they held the whales in, the Voyage Home. You don't remember Scotty deciding to use transparent aluminum? Anyway, go watch The Voyage Home. They had to save the whales. Metals have free electrons and that's why they don't transmit light. They actually absorb light with all those free electrons.