APS News


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 H2O compared with D2O, 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

Back Page Labeled Propaganda

I am appalled that Wasif Syed’s Back Page (APS News, January 2009), which ostensibly purported to deal with science and nuclear policy in Pakistan, was craftily turned into shameless propaganda.

The article suggests that Abdul Qadir Khan, the illegitimate father of the Pakistani nuclear program, is a “scientist whose abilities are not in doubt.” Regardless of ethnocentric and personal biases, we can all agree that scientific ethics and professional integrity are two of the greatest litmus tests for every scientist. For a scientist of his supposed caliber, why was Khan compelled to steal centrifuge technology from URENCO, with whom he worked in the Netherlands, to develop Pakistan’s nuclear program? Pakistan, after all, has produced eminent physicists such as Nobel Laureate Abdus Salam, after whom the International Centre for Theoretical Physics in Trieste, Italy is named!

The clandestine nuclear program in Pakistan, and its fly-by-night operators, are the greatest source of nuclear proliferation in the history of nuclear weaponry, having sold nuclear technology to Iran, Libya and North Korea, all under the watch of General Pervez Musharraf. Pakistan is the epicenter of global terrorism and nuclear proliferation, a most dubious perfecta. Pakistan, a failed state, is every terrorist’s dream for obtaining nuclear weapons as clandestinely as they were developed by the state itself.

Jai A. Pathak
Arlington VA

Blunder May Not Have Been Einstein's

This Month in Physics History (APS News, January 2009) asserts that in 1929, “The cosmological constant looked unnecessary, and Einstein then abandoned it, calling it his greatest blunder.” Actually, we do not know that Einstein called the cosmological constant a “blunder” much less his greatest one; we know only that George Gamow, in his 1970 biography, “My World Line,” asserted “Much later, when I was discussing cosmological problems with Einstein, he remarked that the introduction of the cosmological term was the biggest blunder of his life.”

Gamow, of course, was a brilliant physicist, but besides his physics brilliance he was known for his waggishness, for example adding Hans Bethe’s name to a paper by Ralph Alpher and himself, in the middle position, to make the authors’ names sound like the alpha, beta, gamma that open the Greek alphabet. Gamow should not be considered the most reliable source, particularly not for a specific quotation, and it appears to me that the “blunder”comment may not have really been made by Einstein.

Jay M. Pasachoff
Pasadena, CA

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Editor: Alan Chodos