[00:00.00]W: There’s something I’ve always wondered about. [00:01.77]OK. A glass of water is crystal clear. [00:04.74]A bathtub full of water looks faintly blue. [00:07.38]And the water in a swimming pool is light blue at the shallow end and a darker color blue at the deep end. [00:12.67]Does water have a color? I mean, its own true color?
[00:16.57]M: Believe it or not, it does.
[00:18.51]W: OK. Then what is it?
[00:20.98]M: It’s blue, like a piece of blue glass. [00:23.36] And like glass or any other relatively transparent substance, the thicker it is, the deeper the color. [00:29.93]That’s why a swimming pool is a darker blue at the deep end.
[00:33.54]W: OK. Then what l learned as a kid was wrong. [00:36.57]In grade school I was taught that water doesn’t have a color, that it’s clear, and clear isn’t a color. [00:42.03]But my own eyes tell me otherwise. [00:44.09]So, why is this? I mean, why is water blue, like blue glass?
[00:49.09]M: It’s because water transmits light of every color, but it’s clearest for colors with shorter wavelengths. [00:56.47]That’s why its peak transparency is in the blue-green part of the light spectrum.
[01:01.43]W: That explains why most of the time it looks blue or green.
[01:04.66]M: Well, there are actually lots of factors that affect the color of water.
[01:08.69]W: Such as ... what, like how deep the water is?
[01:12.78]M: Right. And it’s easy to see why. [01:15.25]It’s because light from water comes from three places: the top, the middle, and the bottom of the water.
[01:21.48]W: I’m not sure I understand.
[01:23.67]M: Think of it like this. [01:24.85]Together, all three parts of the water—middle, and bottom—make up the blend of light that we see. [01:31.53] Under certain circumstances, one may dominate the others. [01:35.44]For example, in shallow water, light from the bottom is most important. [01:39.90]In deep water, very little light comes from below, so reflections from the surface are the brightest.
[01:46.62]W: So, that means, in a shallow puddle, the bottom is well lit, so it sort of contributes more to the scene. [01:52.91] I mean, it affects the overall color more than ... say uh ... the invisible bottom of the ocean?
[02:00.02]M: Uh ... that’s right. Because a shallow puddle is relatively transparent, the water adds virtually no blue light of its own.
[02:07.36]W: And muddy water looks brown because...?
[02:10.43]M: Because the light is scattered, primarily from the suspended sediments near the surface. [02:15.18]That’s another factor that affects the color of water—sediments. [02:18.98]Even the cleanest, purest mountain lake contains a lot of tiny particles that scatter light.
[02:24.66]W: That’s because mountain lakes have a lot of silt from the glaciers, right?
[02:28.24]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. [02:37.52]Glacier-fed streams are usually sort of a milky blue, and hot springs are sometimes yellow because of the sulfur suspended in the water. [02:46.62]Some water is green and some is brown, depending on the mineral content.
[02:51.45]W: This is really interesting. [02:52.85] I’m glad I asked you about this.
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