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OK, lets formulate it differently and say water works as a blue passing / red restricting filter.

It is actually observable. Just do a dive in a swimming pool with white light (maybe even at night) and a pool with a metal casing or white tiles. The further you look, the more blue it is.

So why is this so? Why is water not transparent?

And how does it happen on a molecular, subatomar or electron shell level?

Do 'red' waves become absorbed and retransmitted as 'blue' light waves or do only the 'red' waves get absorbed and the 'blue' can pass?

Can Maxwell help?

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I can't give a complete answer because it seems there is still some research ongoing.

Unlike what most people have been taught, water is not colorless. At least, large masses of water will be seen blue, such as the sea or a swimming pool.

enter image description here

(Left: tube filled with (light) water. Right: tube filled with heavy water.)

The fact is that water absorbs mostly the red wavelength. The reason is vibrational , unlike most materials which have its color thanks to electronic absortion, Rayleigh scattering, diffraction...

This vibrational mechanism means that water molecules will be excited into higher frequency vibrational modes (like a guitar string). This is not the case of deuterated water, as you can see.

enter image description here

Note that in the cases of the sea, there are more reasons involved, like scattering due to impurities and reflection.

You can read more here: http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water, http://www.webexhibits.org/causesofcolor/5B.html and http://www.dartmouth.edu/~etrnsfer/water.htm.

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  • $\begingroup$ Right tube is not empty — it's filled with D₂O. $\endgroup$ – Ruslan Mar 14 at 10:21
  • $\begingroup$ @Ruslan The right tube is empty. From the linked reproduction and the original article: "The large tube volume and a limited budget precluded checking to see if light transmitted through a D2O filled tube was indeed white, as expected." The reproduction linked adds a comment and image not in the original paper claiming that the image depicts a tube filled with D2O, but I doubt that they would do that shortly after their paper if they were really concerned about the cost. $\endgroup$ – Tyberius Mar 15 at 1:09

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