‘because it is a liquid’ is a nonsense answer; as there are solids, e.q. quartz crystals and diamond which are perfectly ordered crystals and yet are perfectly transparent; and many fluids that are strongly opaque.
In short, the phase of a material, gaseous, liquid or solid has no direct relation to its transparency. The question is whether there exist a) internal surfaces, like in a bag of sugar, which refract, reflect and disperse light; or b) energy transitions thaty can be triggered by the energy of passing photons to absorb them.
I completely agree with Bart van Herk, Please, ‘A Liquid’
The real answer, is evolution, the human, and many other eyes have evolved to see through different materials, particularly ones we live in like water and Air. Glass, or rather silica glass is not completely transparent, we add other elements. ‘clearing agents’ that make it, ‘transparent to OUR eyes’ that’s not to say that it’s transparent to every part of the spectrum only to that part of the spectrum we can focus with our eyes. Hence the reason crystals and other naturally occurring elements can be transparent is because we evolved to see through them. Not the other way around or by the state of matter they currently reside in.
You’re kidding, right? Whether or not a photon is absorbed, scattered, or transmitted by a material is independent of the human eye, or even of whether or not humans ever evolved.
If you are interested in the physics, there is a discussion on the “Glass: what is it” thread.
You are missing the point to what Branedy is saying. The point is that “transparent” simply means that a particular set of electromagnetic radiations can pass through the material. Which set? The set the human eye evolved to see. Such a material is not “transparent” necessarily to something that sees in the infrared spectrum.
So the post was not trying to establish a linkage between the physical properties of materials and human evolution, but rather between the meaning of the word “transparent” and the fact it refers to a property only important because of how humans see.
I don’t know, I think Branedy’s point is unclear. You are correct that we call something transparent because we can see it. But from an underlying principle standpoint, it is the physics that rules.
The physics defines whether photons go through, get reflected, or are absorbed. So it is the physics that defines if we could have the chance of seeing through the object. Yes, our eyes evolved to detect certain wavelengths of light. Yes, it is the photons at those wavelengths that we are concerned with. But that’s more of a description of what transparent means, not how it occurs. How it occurs is through the physics.
And nothing that Branedy says disagrees with that. The whole point of the post was not to explain what makes a thing transparent (the physics), but why we call a thing transparent (the fact that it has this physical property).
In the physics community, at least, the word “transparent” is not restricted to visible light. It is common to speak of material as being transparent to infrared, UV, x-rays, or whatever.
The etymology, though, is on the side of visible light, from the Latin transparere (trans- = “beyond” + parere = “be visible”). And the OED gives visible light the first definition, with transparency to other radiations in the second definition.