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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I read comments on a similar question as this. And it boiled down to our eyes evolved to be able to see through water since it was advantageous to do so. We evolved from sea creatures so seeing in water was a huge benefit. Think about bees. I believe certain bees, if not all, can see in ultraviolet. It is an evolutionary advantage.
You can see through gold in a solid state. The original visors on NASA space helmets were coated in gold foil. Gold is so malleable you can press it thin enough to where you can still see through it but it reflects a lot of light. I think the newer visors use vapor deposition but still use gold.
More to read here: https://spinoff.nasa.gov/spinoff1997/hm2.html
Materials tend to be transparent in certain wavelengths of light. Water’s transparent range coincides with visible light for humans. This is not a coincidence, being able to see through water is important for aquatic creatures, which is where the eye originally developed.
For example, if you go into the infrared range, many types of glass becomes opaque but silicon becomes transparent.
It isn’t about density. Water is actually fairly dense, especially in liquid form. Transparency, refraction, reflection, and absorption of light is extremely complex. But it mostly boils down to the chemical properties of a material.
Remember that the part of an atom that directly interacts with the world around it is the outer electron shell. Keep in the back of your mind that this, and everything else I’ll be saying here is a heavy simplification. That outer electron shell may absorb or reflect light, or may largely ignore it. That depends on the atom, the molecule, and the light. If it reflects light, the molecules may be structured in such a way that light tends to bounce through, rather than back the other way.
Pretty much all materials are transparent to some kinds of light, allowing a lot of it though. For example, your body allows a lot of x-rays though, except for your bones. (That’s how x-rays work: they shine a light that projects x-rays like the flash on a camera)
Water is transparent to some light, and blocks other kinds of light (it absorbs microwaves very well, for example). This is because different light has different wavelengths. Visible light mostly passes though water without being absorbed or reflected, but shorter and longer wavelenths don’t.
Our eyes evolved to take advantage of the sunlight that wasn’t absorbed in shallow water, so ‘light that isn’t absorbed by the earth’s atmosphere or water’ became our visual spectrum.
Imagine light as a beach volleyball. You can have a densely packed field of amateurs which not a single one capable of blocking the ball. Likewise a single pro may easily stop it.
It’s all about proficiency.
Similarly, light can be blocked by interacting with electrons of atoms. But depending on some factors, those electrons can only absorb light with a specific energy. In gold and other materials, this energy falls in the range of visible light. In case of water it falls in the range of infrared while visible light is not blocked.
Light typically interacts with materials by giving energy to the electrons in the material. Electrons aren’t allowed to have just any amount of energy, so light can only interact with a material if some electrons in the material are allowed to gain as much energy as the light has to offer. The amount of energy light has to offer is related to the color of the light.
Water has a few electrons that can accept red light, so large bodies of water can look slightly blue (because when you take away red light, blue is left). But other than that, water doesn’t interact with light very much. By contrast, metals (including gold) have some electrons that have a lot of freedom of movement, so they can interact with any color of light and are shiny. These electrons with freedom of movement also carry heat and electricity through metals, and hold the atoms together when you bend a metal object.
There’s actually a few metals that are less dense than water, and they’re still opaque and shiny. Lithium, sodium, and potassium would all float in water, but they’re also so reactive they’ll catch fire if they get wet, so you shouldn’t try that at home.
Also, if you have ammonia gas without any water and get it so cold it condenses, it’ll form a clear liquid that looks like water, and if you run electricity through the liquid ammonia, the moving electrons have a lot of freedom of movement so the liquid turns shiny like a metal. But ammonia gas is poisonous, so that’s another experiment you probably shouldn’t try at home.
>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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