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W: There’s something I’ve always wondered about. OK. A glass of water is crystal clear. A bathtub full of water looks faintly blue. And the water in a swimming pool is light blue at the shallow end and a darker color blue at the deep end. Does water have a color? I mean, its own true color?
M: Believe it or not, it does.
W: OK. Then what is it?
M: It’s blue, like a piece ofbhie glass. And like glass or any other relatively transparent substance, the thicker it is, the deeper the color. That’s why a swimming pool is a darker blue at the deep end.
W: OK. Then what I learned as a kid was wrong. In grade school I was taught that water doesn’t have a color, that it’s clear, and clear isn’t a color. But my own eyes teI1 me otherwise. So, why is this? I mean, why is water blue, like blue glass?
M: It’s because water transmits light of every color, but it’s clearest for colors with shorter wavelengths. That’s why its peak transparency is in the blue-green part of the light spectrum.
W: That explains why most of the time it looks blue or green.
M: Well, there are actually lots of factors that affect the color of water.
W: Such as ... what, like how deep the water is?
M: Right. And it’s easy to see why. It’s because light from water comes from three places: the top, the middle, and the bottom of the water.
W: I’m not sure I understand.
M: Think of it like this. Together, all three parts of the water-top, middle, and bottom-make up the blend of light that we see. Under certain circumstances, one may dominate the others. For example, in shallow water, light from the bottom is most important. In deep water, very little light comes from below, so reflections from the surface are the brightest.
W: So, that means, in a shallow puddle, the bottom is well lit, so it sort of contributes more to the scene. I mean, it affects the overall color more than ... say uh. ... the invisible bottom of the ocean?
M: Uh ... that’s right. Because a shallow puddle is relatively transparent, the water adds virtually no blue light of its own.
W: And muddy water looks brown because...?
M: Because the light is scattered, primarily from the suspended sediments near the surface. That’s another factor that affects the color of water-sediments. Even the cleanest, purest mountain lake contains a lot of tiny particles that scatter light.
W: That’s because mountain lakes have a lot of silt from the glaciers, right?
M：That’s right. Lakes fed by glaciers get their colors from the fine, suspended silt in the water, and also from the reflected light of the sky. Glacier fed streams are usually sort of a milky blue, and hot springs are sometimes yellow because of the sulfur suspended in the water. Some water is green and some is brown, depending on the mineral content.
W: This is really interesting. I’m glad I asked you about this.