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Illuminating the Sunset's Green Flash
15 June 1998 7:30 pm
SAN DIEGO--Many people who claim to see the famed "green flash" as the sun dips below the horizon aren't seeing green at all, but rather are temporarily color-blind from staring at the sun too intently, according to research presented here last week at a meeting of the American Astronomical Society. However, the work also shows that the camera doesn't lie: Bona fide flashes do occur, thanks to mirages in the lower atmosphere.
Most optics texts misinterpret the brilliant green rays of light that sometimes appear at sunset, says astronomer Andrew Young of San Diego State University in San Diego, California. Many authors describe a "green rim" at the sun's upper edge, which supposedly appears when the air refracts the colors of the spectrum at different angles as the sun disappears. That effect does exist, Young says, but it's too tiny to account for flashes. Rather, he says, anyone who spies a flash is seeing a mirage similar to phantom waves of asphalt that hover above a scorching road.
Young simulated the paths that light travels through thick air blankets just above the horizon. He found that two main types of green flash mirages can appear, depending on the viewer's altitude and the temperatures of the air and ground. In both mirages, parts of the sun's image reflect off warm or cool layers of air, and at precise moments, a combination of atmospheric refraction, absorption, and scattering can remove all colors from the sunlight except a spectrally pure emerald green. Most flashes require a flat horizon--often the ocean--and a clear sky. Young's research also revealed a surprising physiological effect: temporary "bleaching" of red-sensitive pigments in the retina from staring at the setting sun. This condition can make a viewer perceive the yellow edge of the setting sun as green. "Most visual reports of the green flash are probably from people seeing the yellow stage," says Young, who has a Web site on green flashes. Watching for the flash as the sun rises is one way to avoid this effect, he says.
Young "is the world's expert on the green flash," says planetary scientist David Lynch of the Aerospace Corporation in El Segundo, California. "He's learning in detail how light propagates through the lower part of the atmosphere," which is difficult to study optically because of the constant changes in that boundary layer, he notes. Lynch also credits Young with broadening public awareness of green flashes. "They can be tricky to see, but they're actually quite common," Lynch says.