QnA Stuffs, Nation! (X)
OK, Phil…SciShow is here for you. Technically, we can’t see water either, unless we’re looking through a bunch of it. But your average glass of water, you aren’t seeing the water in there, you’re seeing two effects:
1. The surface where the water and the air meet (or the water and the glass). That surface can reflect light. Fish can see the surface of the water as well (it looks different from below, but it’s still visible.) But you can’t really see the water. Also, when water is on a person and they’re all shiny, that’s just because the water on them is all surface and you’re seeing all that surface.
2. The difference in the index of refraction between water and air. Light moves at different angles through different materials. So when you see water, especially in a glass, you’ll see it bending the light differently than air bends light. Fish would not see this, in the same way we don’t notice the index of refraction of air.
However, they DO see AIR in the same way WE see WATER! They would see the surface between the two and they would notice the difference in refraction between the two substances and note that “air” is a thing that they can see just like we imagine water as something we can see. Bubbles, y’know…
Now, there are two other things you might think. One, that you can see water because there’s stuff in it…but that’s just seeing the stuff…like you wouldn’t say you could see air just because there’s smoke or a cloud…you’re just seeing stuff IN the air.
And last…water actually does absorb light, so if you look through a lot of it, you /can/ see it. However, this is also true of air, it’s just that air absorbs less light (much less, if there’s no water vapor in it.) So, in the same way that we can see air if we’re looking through enough of it (a very far away mountain will appear diluted and blue) fish would also be able to see water.