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Why Is Water Blue?

by Eric Herman July 24, 2012 10:54 AM

Eric HermanWhen you ask most anyone, “Why is water blue?” they are likely to say it’s reflecting the color of the sky. Given that we know that liquid water is a highly reflective material, and given that a cloudless sky in the daytime is, indeed, blue, that conventional wisdom seems reasonable. 

I was recently discussing this issue with my friend, David Knox, president of Lightstreams Glass Tile, a manufacturer of glass-tile products that have unique optic qualities based on Knox’s prior career inventing LASER applications for high tech industries. David’s a fascinating guy, I daresay one of the true geniuses I’ve ever known. As is the case with most discussions we have, he offered insights that go well beyond my common understanding of science and nature. In this case, as he explained, the blueness of water is a far more complex and intriguing subject than I had ever considered. 

He referred me to a fascinating paper written by researchers at Dartmouth College’s Department of Chemistry, Why is Water Blue by Charles L. Braun and Sergei N. Smirnov. http://www.dartmouth.edu/~etrnsfer/water.htm.

According to Braun and Smirnov, the reason water is blue is largely due to the unique way it reflects light. They write, “Because the absorption, which gives water its color, is in the red end of the visible spectrum, one sees blue, the complementary color of red, when observing light that has passed through several meters of water. This color of water can also be seen in snow and ice as an intense blue color scattered back from deep holes in fresh snow. Blue to blue-green hues are also scattered back when light deeply penetrates frozen waterfalls and glaciers.”

But what about the blue color of the sky? We’ve all seen lakes, rivers and oceans that appear gray under cloudy skies. Is all of this to say that the sky has no effect? No, say the researchers; reflected light is a factor, too. The color of water is a combination of reflection, the absorption of red wavelengths and a third factor, the material suspended within it.  

“Light scattering by suspended matter is required in order that the blue light produced by water’s absorption can return to the surface and be observed. Such scattering can also shift the spectrum of the emerging photons toward green, a color often seen when water laden with suspended particles is observed.” 

To further complicate matters, there’s a fourth factor in play. “The relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle.”

For what it’s worth, after reading this fascinating discussion, I’ll never look at an azure sea or swimming pool with the same eyes. Why is water blue? Turns out, it’s complicated.

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