Why can you see through H2O as both a liquid and a solid but you cannot see through something like Au (gold) in a solid state? Is this a matter of density?
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>Is this a matter of density?
Often yes, but in this case it’s the density of electrons. The interaction of light with matter is *incredibly* complex. People claiming that all materials have some wavelengths at which they are transparent are… misguided. For a lot of materials, one very important number is what’s called the plasma frequency. That’s the frequency at which free electrons in the material will jiggle if you whack them, and basic physics will tell you that that frequency is set by the density of those electrons. If you shine light with lower frequency into a plasma then the electrons adjust to cancel out the electric field in the light wave, which means the light doesn’t go through and instead is reflected. If you shine higher frequency light in, the electrons don’t have time to adjust, and light goes through. Conductors have free electrons (that’s why they conduct), and for metals, the plasma frequency is typically in the UV. Visible light then will just bounce off them, and that’s why metals are reflective. Sea water also conducts, but has a much lower density of things that conduct, so its plasma frequency is much lower. That means that visible light can go through, but radio waves don’t (that why we use sonar instead of radar to map the sea floor).
Of course, molecules can also have transitions that are good at absorbing certain wavelengths of light (look up “atomic spectra” to see some examples), but water happens to not have transitions in the region our eyes are sensitive to. I’ll also add that our eyes are sensitive to the frequencies where the sun’s output peaks, and it’s really mostly a coincidence that water happens to be transparent in that range. Of course, it would be extremely hard for life to develop on a planet where all the incoming sunlight was blocked by water (which could happen if the sun was a lot smaller and cooler, emitting in the IR), so maybe it’s not a coincidence that life developed where the peak of solar radiation isn’t totally blocked by water.
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