The Real Reason Water is Blue
Dear Editor,
“This Month in Physics History” is my favorite part of
APS News, and the
February column about Raman was particularly good. It is easy to believe that the question “why is water blue” led Raman eventually to his great discovery. What is not mentioned in the article is subsequent discussion of this question, which has shown that water is selectively absorbing in the red. This is familiar to divers, who experience the ghostly blue illumination that sunlight provides at depths of 10 meters or more. Raman scattering is not a significant part of the answer to this wonderful puzzle. Impurities in water are not either. There are no electronic transitions in pure water until the ultraviolet, and vibrational transitions are surely deep in the infrared, so what is the explanation? The answer is a great surprise to students of optical properties of matter. It is so interesting that I think readers of
APS News should be fascinated to hear it. Fourth harmonics of the symmetric and antisymmetric “O-H stretching vibrations” lie just at the lower end of the visible energy spectrum, and are responsible for the weak absorption. It is the only familiar situation where vibrations are the primary cause of visible coloration, although other cases (liquid ammonia, for example) could be found if desired.
I am not sure to whom this explanation should be attributed. Confirmation and popularization of the vibrational mechanism was done by Charles Braun and Sergei Smirnov. They have a delightful paper in the
Journal of Chemical Education (v.70(8), p.612, 1993), which is available on
Prof. Braun’s web page at Dartmouth. There you can see the spectrum of liquid H
2O compared with D
2O, which gives convincing evidence of the vibrational mechanism. Braun and Smirnov cite various earlier authors, the earliest being W. A. P. Luck (1965), and several good pedagogical treatments, especially by C. F. Bohren. Paraphrasing Bohren, they mention that “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.”
Yours truly,
Philip B. Allen