So water is blue. Why?

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So water is blue. Why?

Postby Unlocked » Sat Mar 18, 2017 5:31 am UTC

I get that water can look blue because of reflections from the atmosphere and whatnot, but it's also actually very slightly blue without that external factor. I get that impurities can cause colors, but I'm pretty sure this also applies to pure water, so I was wondering why? I'm not looking for answers like "water just happens to absorb all of the other colors of light." Why is it blue on a particle/sub-atomic particle level (if the explanation has to go that far down)?

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Re: So water is blue. Why?

Postby Sableagle » Sat Mar 18, 2017 12:36 pm UTC

This may be relevant: http://www.chemguide.co.uk/atoms/proper ... ctrum.html

It has to do with particular colours of light on a sub-atomic level, anyway.

This is all about it: http://www1.lsbu.ac.uk/water/water_vibr ... ctrum.html
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Re: So water is blue. Why?

Postby Soupspoon » Sat Mar 18, 2017 1:49 pm UTC

Unlocked wrote:I get that impurities can cause colors, but I'm pretty sure this also applies to pure water, so I was wondering why?

This looks "flipwise". Colours aren't caused, outside of actual luminescence/incandescence, but are merely preserved (or not) from whatever actual illumination is passed through.

It is that it absorbs other wavelengths of light. Not much, as it isn't noticeably opaque in any visible wavelength, but more is absorbed away from the blue than in the blue. Random technical link with some relevant spectra and stuff about quanta.

Water with a dye as an impurity is coloured by the influence of that dye (absorbing the colours "that it is not a dye for"1).
Water totally free of impurities is coloured by the influence solely of the water. And water looks very slightly blue because water is very slightly blue. Everything is (at least) slightly something, in colour. Except nothing (and even then..?). And so everything has colours, even the air (before we even get to the Rayleigh Scattering). But as we have developed with no good reason to perceive the relative presence/absence of light beyond the visible spectrum2, we do not have an acuity that permanently perceives any pigmentation or opacity in the normal, clean typical mixture of atmospheric gases that is our air, as habitual inhabitants of the atmosphere.

Liquid water happens to just be marginally more absorbing (the density helps) and skewed to give us blue. Which, under normal daylight lighting conditions, doesn't make the average glass of reasonably drinkable water look much other than as transparent as air, or the glass. In the middle-depths of the ocean, red light (deficient in the ambient light filtered down from the surface) is mostly deprecated for the denizens of those depths, as they favour the faint blue of the remaining sunlight and of the bioluminescent photophores they have developed (a red fish is a black fish, nofmally, as far as their visibility is concerned). They probably consider their watery environment "unpigmented" and, to their limited experience and specifically attenuated eyes, as effectively colour-free as air.

(Also necessarily leaping the barrier of understanding that is Wittgenstein's Lion...)


In summary, as Yoda (nearly) said: Blue, or blue not. There is no "why"...


1 The dye is a dye because it does this, it is not doing this because it is a dye. Or, to put it another way: there is no physical "is a dye" property to a dye, just physical properties to substances such that some substances are very effective at creating "is a dye" effects (which includes affinity to the desired substrate and/or miscibility in suitable medium, as well), so we call them dyes. (Or pigments, etc, depending in context.) It's all a little metaphysical.

2 Again, so-called-visible because we perceive it - not perceived because the universe has a label for "visible light" that we then slavishly adhere to in order to cut down on the cosmic paperwork...

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Re: So water is blue. Why?

Postby Zamfir » Tue Mar 21, 2017 8:21 am UTC

You can find more details in the links above, but I'll give a simplified picture to start with:

A passing photon only gets absorbed by a molecule, if the energy of the photon is exactly enough to shift a molecule to a higher vibration pattern. Not too much, not too little. If we want to know which frequencies of light get absorbed, we have to look at the vibration patterns ("modes") of water molecules.

In the neighbourhood of visible light, the most important mode is "stretching". That means that the hydrogen atoms vibrate towards and away from the oxygen atom. If a photon with (roughly) 3000nm wavelength comes near a water molecule, it can be absorbed and the molecule vibrates faster.

If it was just this phenomenon, we wouldn't notice it at all. For one, our vision ends below 750nm. And two, a single sharp band of absorption would just filter out that exact frequency within a fraction of a milimeter of ater depth, and nothing more would happen. So we have to look at further complications of the story to explain the blue colour.

One complication: the molecule can also move up multiple steps in one go, if a higher-energy photon comes near. Higher energy means shorter wavelengths. For example, a 1900nm photon leads to 1 "stretching" mode combined with 1 "bending" mode. With a 1400nm photon, the moleculecan move up 2 steps in stretching vibration, etc. As result, there are absorption bands at shorter wavemegths than the main 3000nm band. It's more rare for this to happen, and these "combined mode" bands absorb less strongly as they require more modes together.

Now we get all the way to 4 steps stretching, or 3 stretching plus 1 bending. Those modes require photons just at the edge of the visible spectrum, around 700nm. These absorb a million times less than the main peak at 3000nm.

The second complication: in liquid water, every molecule is attached to its neighbours through hydrogen bonds. Those bonds modify the vibration modes of the molecule, depending on the position of its neighbours. As result, you do not see sharp absorption peaks at precise frequencies, but broad bands around those frequencies, because every molecule is "tuned" slightly different. This effect fismears out the peaks, filling in the gaps between them.

Put that together, and you get this picture (note the logarithmic scales, and the "vis" visible spectrum colours below):
Look at the peak at 3000nm (3um), at the two smaller peaks to the left for 1900nm and 1400nm mode. Then a peaky decline further to the left, due to all the more complicated vibration modes. The bottom of the absorption valley is right in our visible blue light, so that's why water is blueish. Even further to left, in the ultraviolet, you get absortion due to electrons within the atoms changing their energy level (often causing the molecule to break up).
Image

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Re: So water is blue. Why?

Postby Xanthir » Tue Mar 28, 2017 6:13 pm UTC

Soupspoon said it, but with a lot of text preceding it, so I'll answer it simply:

Water is blue because that's the color of water. Same as (red) apples are red because that's their color. All the same mechanisms that give other physical objects their color work on water too, and the details are such that water ends up blue-colored. (There's some other interesting effects that can intensify the blue-ness, but mostly it's just that H20, in bulk, is blue.)
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Re: So water is blue. Why?

Postby LaserGuy » Fri Mar 31, 2017 11:54 pm UTC

I wonder if Randall was reading this thread.

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Re: So water is blue. Why?

Postby chenille » Mon Apr 03, 2017 6:24 pm UTC

Xanthir wrote:Water is blue because that's the color of water. Same as (red) apples are red because that's their color.

Yes and no. Yes, water is blue as a material, but it's legitimate to ask why and the answer isn't quite the same as why the flavonoids in apples are red. Lots of substances get their visible absorption from the energy levels for electrons – changes in those tend to correspond to ultraviolet, but become visible when some levels are closer together, for instance by having lots of conjugated double bonds like in those plant pigments, or having a transition metal ion in an environment that splits the d orbitals like in copper sulfate.

Whereas the color of water is what Zamfir said. It gets its visible absorption more from the energy levels for molecular vibrations – changes in those tend to correspond to infrared, but become visible when some levels are farther apart, and that happens here in part because of the way hydrogen bonding sticks the molecules together. So not only is there an answer you can give beyond "it is", but I think it's one that tells you something interesting about water.

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Re: So water is blue. Why?

Postby Xanthir » Mon Apr 03, 2017 9:24 pm UTC

Right, but as Randall said in his comic afterwards, the physics-based explanations are interesting, but they're the same level of interesting as asking why anything else is the color it is. For some reason we tend to elevate the question of water's/air's color to a special place, tho, where "it's just blue" is no longer an acceptable answer, and we instead reach for the physics answers right away.
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Re: So water is blue. Why?

Postby chenille » Mon Apr 03, 2017 9:59 pm UTC

Well, air is special; it's not simply one color like water or an apple, as the same material that makes the daytime sky look blue makes the sunrise look red. The thread for that comic already has lots of discussion on the ways "air is blue" is misleading, which I won't repeat here. What I will say is I'm not sure why there's so much dislike for providing the physics explanations when they come up.

Personally I think the origin of colors is a fascinating topic. Even if it weren't necessary to explain the different colors air creates, I'd still like to share scattering as a neat example that just about anyone can understand in general terms. And here we are on a science forum, with a question explicitly about why water is blue on a particle level, and you still wanted to leave it at "water just is" instead of looking at details. It's really strange to me that people otherwise interested in how the world works so often prefer to shut down on these questions.

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Re: So water is blue. Why?

Postby Xanthir » Tue Apr 04, 2017 5:05 pm UTC

I didn't "want to leave it at that"; plenty of preceding posters had already given technical explanations, and I didn't disagree with them. Maybe chill for a minute, yo.

As I explained, most people reach for the physics explanations *immediately*, when there's a simpler answer - that blue is just the color of water/air. Jumping straight to complicated explanations makes water/air look *mysterious*, like it's somehow special and different from any other colored thing. But it's really not - there is some unique and interesting physics going on, but honestly that's true of nearly all colored things.

After establishing that "water is just blue-colored, like grass is green-colored" as a base, *then* you can go about wowing them with the interesting science part. Because the scattering effect is really cool, and *is* different from the reflection/absorption effects that give most things colors (tho absorption is *part* of why air is blue). Just trying to break the tradition of blinding people with science immediately.
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Re: So water is blue. Why?

Postby Zamfir » Thu Apr 06, 2017 6:08 am UTC

As I explained, most people reach for the physics explanations *immediately*, when there's a simpler answer - that blue is just the color of water/air. Jumping straight to complicated explanations makes water/air look *mysterious*,

Though in this case, the OP asked for this jump, in so many words:
Unlocked wrote: I'm not looking for answers like "water just happens to absorb all of the other colors of light." Why is it blue on a particle/sub-atomic particle level (if the explanation has to go that far down)?


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