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THE SCIENCE of the GLORY Author(S): H. Moysés Nussenzveig Source: Scientific American, Vol. 306, No. 1 (January 2012), Pp. 68-7

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THE SCIENCE of the GLORY Author(S): H. Moysés Nussenzveig Source: Scientific American, Vol. 306, No. 1 (January 2012), Pp. 68-7 THE SCIENCE OF THE GLORY Author(s): H. Moysés Nussenzveig Source: Scientific American, Vol. 306, No. 1 (January 2012), pp. 68-73 Published by: Scientific American, a division of Nature America, Inc. Stable URL: http://www.jstor.org/stable/26014160 Accessed: 07-08-2017 14:37 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms Scientific American, a division of Nature America, Inc. is collaborating with JSTOR to digitize, preserve and extend access to Scientific American This content downloaded from 81.194.22.198 on Mon, 07 Aug 2017 14:37:28 UTC All use subject to http://about.jstor.org/terms PHYSICS THE SCIENCE OF THE GLORY One of the most beautiful phenomena in meteorology has a surprisingly subtle explanation. Its study also helps to predict the role that clouds will play in climate change By H. Moysés Nussenzveig HINZ CLAUDIA LAUGHON; HINZ; BARBARA CLAUDIA JURVETSON; STEVE LEFT: TOP FROM CLOCKWISE 68 Scientific American, January 2012 This content downloaded from© 2011 81.194.22.198 Scientific Americanon Mon, 07 Aug 2017 14:37:28 UTC All use subject to http://about.jstor.org/terms January 2012, ScientificAmerican.com 69 This content downloaded from 81.194.22.198 on© Mon,2011 Scientific07 Aug 2017 American 14:37:28 UTC All use subject to http://about.jstor.org/terms H. Moysés Nussenzveig is an emeritus professor of physics at the Federal University of Rio de Janeiro and a winner of the Max Born Award of the Optical Society of America. He has developed novel theoretical approaches to a broad range of optical phenomena and is currently directing research in cell biophysics. n a daytime flight pick a window seat that will allow you to locate the shadow of the airplane on the clouds; this requires figuring out the direction of travel relative to the position of the sun. If you and ultimately received a No- are lucky, you may be rewarded with one of the bel Prize for its invention.) most beautiful of all meteorological sights: a multi- The shadow of the observer or the airplane plays no role in colored-light halo surrounding the shadow. Its iri- creating a glory. The only rea- descent rings are not those of a rainbow but of a different and more son for their association is that O shadows mark the direction subtle effect called a glory. It is most striking when the clouds are exactly opposite to the sun in closest because then it dominates the whole horizon. the sky, signifying that the glo- ry is a backscattering effect, in If you are a mountain climber, you may also see a glory soon which sunlight gets deviated by nearly 180 degrees. after sunrise, around the shadow your own head casts on nearby You would think that such a well-known effect, involving op- clouds. Here is how it was described in the first reported observa- tics, a venerable branch of physics, would surely have been ex- tion, published in 1748 and made a decade earlier by members of plained long ago. Yet for scientists this “phenomenon which must a French scientific expedition to the top of Pambamarca in what be as old as the world,” in the words of the 1748 report, remained is now Ecuador: “A cloud that covered us dissolved itself and let a challenge for centuries. Rainbows are themselves far more com- through the rays of the rising sun.... Then each of us saw his plex than introductory physics textbooks would lead one to be- shadow projected upon the cloud.... What seemed most remark- lieve. Still, rainbows are considerably simpler than glories. able to us was the appearance of a halo or glory around the head, In principle, both glories and rainbows are explained using a consisting of three or four small concentric circles, very brightly standard optics theory that was already available early in the 20th colored.... The most surprising thing was that, of the six or seven century, when German physicist Gustav Mie wrote down an exact people who were present, each of them saw the phenomenon mathematical solution of how water droplets scatter light. The only around the shadow of his own head, and saw nothing around devil, however, is in the details. Mie’s method involves the sum- other people’s heads.” mation of terms called partial waves. The summation includes in- Scholars have often suggested that the halo around the heads finitely many such terms, and even though only a finite number of deities and emperors in eastern and western iconography may matter in practice, Mie’s method still requires evaluating hun- have been a representation of a glory. Samuel Taylor Coleridge’s dreds to thousands of mathematical expressions, each of which is celebrated poem “Constancy to an Ideal Object” is an allegorical rather complicated. Put the formulas into a computer simulation, tribute to it. In the late 19th century Scottish physicist C.T.R. Wil- and they will give the correct result but will provide no insight son invented the cloud chamber in an attempt to reproduce the into the physical effects that are responsible for the phenomenon: phenomenon in the laboratory. (Wilson failed, but he quickly re- the Mie solution is just a mathematical “black box” that, given alized that he could use his cloud chamber to detect radiation certain inputs, generates an output. A remark attributed to phys- IN BRIEF Looking down on a cloud from a mountain or an air- croscopic water droplets that compose clouds, but in that missed a droplet can still transfer energy into it. plane, sometimes you can spot a glory: rings of col- the case of glories the physics is more subtle. The understanding gained from glories is helping ored light around your shadow or the plane’s. The light energy beamed back by a glory originates climatologists to improve models of how cloud cover As in a rainbow, the colors are produced by the mi- mostly from wave tunneling, which is when light rays may contribute to or alleviate climate change. 70 Scientific American, January 2012 This content downloaded from© 2011 81.194.22.198 Scientific Americanon Mon, 07 Aug 2017 14:37:28 UTC All use subject to http://about.jstor.org/terms ics Nobel laureate Eugene Wigner is apt: “It is very nice that the ray from the droplet’s center, called the ray’s impact parameter. computer understands the problem. But I would like to under- The calculations involved in Mie scattering from droplets of a stand it, too.” Blind faith in brute-force number crunching can sufficiently broad range of sizes are forbiddingly complex with- also lead to incorrect conclusions, as will be shown. out a high-speed computer, and it was not until the 1990s that In 1965 I began to develop a research program to provide, supercomputers began to be fast enough to give realistic results among other things, a full physical explanation of the glory—a over the broad range of droplet sizes found in clouds. Research- goal that, with the help of several collaborators along the way, ers needed better ways to grasp what was going on. was finally completed in 2003. The answer involves wave tunnel- Hendrik C. van de Hulst, a pioneer of modern radio astrono- ing, one of the most mystifying effects in physics, which Isaac my, provided the first significant insight into the physical expla- Newton first observed in 1675. Wave tunneling is the basis of one nation of glories in the middle of the 20th century. He pointed type of modern touch screen, employed in computers and cell out that a light ray that entered a droplet very close to the drop- phones. It is also important in the notoriously complicated—and let’s edge might follow a V-shaped trajectory inside the droplet, still incompletely solved—problem of determining how atmo- bouncing off at the back, and return almost exactly in the same spheric aerosols, which include clouds but also dust and soot, direction that it came from. Because droplets are symmetric, contribute to climate change. among the bundle of parallel rays coming from the sun the fa- vorable impact parameter would occur not just for one ray but WAVES AND PARTICLES for a whole circle’s worth of rays all at the same distance from over the centuries physicists have offered several explanations the droplet’s center—a focusing effect that would dramatically for glories that proved to be incorrect. At the beginning of the enhance the backscattering. 19th century German physicist Joseph von Fraunhofer pro- The explanation sounds clear-cut, but unfortunately it had a posed that sunlight that is scattered—that is, reflected back— serious snag. As a ray entered and exited the droplet, it would from droplets deep within a cloud would become diffracted by bend via refraction. But the refractive index of water is not droplets at the outer layers. Diffraction is one of the wavelike large enough to scatter a ray back in the same direction after features of light, enabling it to “go around corners,” just as sea just one internal reflection. The best that water can do is send waves can negotiate an obstacle such as a vertical beam and proceed as if the obstacle had not been there at all.
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