I'm not sure when I first heard the term "green flash," or how many evenings I sat on the beach at sunset, hoping to see the legendary gleam. But what I clearly remember is the moment I first witnessed it wink at me across Florida's gulf waters.
Driving back to Naples Bay after a successful afternoon of grouper fishing, several friends and I debated this sunset phenomenon and were skeptical. We'd heard that just as the sun slips over the horizon on a clear night, it can send up a spark of green. But none of us had ever seen it. Some beachgoers spend a lifetime looking for it. Others say it doesn't exist, that it's an optical illusion.
So as we puttered along the shoreline toward Gordon Pass, we watched the reddening sun and hoped that this sunset would be different from hundreds we'd seen before. We counted down the seconds and held our breaths as the last bit of red sunlight fell behind the water's far edge.
And then, there it was, just as I had imagined: a brilliant emerald-green flash. It flared up for a few splendid moments, then disappeared almost before we could shout. Glad to have shared this magical moment with friends, I was even happier that our debate on the subject had ended.
Some claim that references to the green flash date as far back as 2500 BC. They cite diagrams on an Egyptian pillar depicting the sun as a semicircular disc painted blue at the top and green at the bottom as evidence.
Literature hints at the flash. One of the earliest published accounts comes from W. Swan, who first observed the phenomenon in 1865, but did not submit his writings to Nature magazine for almost 20 years. Some speculate that Jules Verne's 1882 romance Le Rayon-Vert (translated "the green ray") sparked a widespread interest in the flash that prompted Swan, and perhaps other observers, to let the public in on their sightings. Verne's account includes a quote he attributes to Scottish legend: "He who has been fortunate enough to behold it is enabled to see closely into his own heart."
But as more witnesses came forward, opinion divided about the flash's true nature. Some argued that it was an optical illusion resulting from fatigue of the retina after looking at the red light of a setting sun. Since green is the complementary color to red, the eye would see green after being dazzled by the bright red sun. But this would not explain the green flash sometimes seen at sunrise, when the retina would not be strained from afternoon rays. Plus, many fleeting flashes have been captured on film, eliminating the possibility that it's merely a physiological phenomenon.
Despite its elusive nature, the green flash can be easily explained by ordinary laws of optics. The atmosphere acts as a prism, separating the sun's light into a spectrum of colors. Each color forms a separate image of the sun. Near sunset, a red rim appears on the bottom and a blue or green rim on the top. When our star is low on the horizon, this separation is greatest.
Since blue light has the shortest wavelength, the top rim scatters the moment it enters the earth's atmosphere, leaving the next color in the spectrum―green. But for us to see this thin rim of green, it must be magnified. When air masses in the lower atmosphere are heated and cooled, under the right conditions, the air itself becomes a lens through which the sun's image (including the green rim) may more than double its original size. Voilà: the green flash.
Despite technical explanations and detailed accounts from around the world, the most intriguing still comes from that purported Scottish legend. This lovely phenomenon is seen only when nature presents prime conditions, but Jules Verne's story says it's only seen by true lovers. At the next sunset, I'll be looking for it.