Have You Ever Seen a Green Flash?

HOW enjoyable it is to bid farewell to another day by gazing at a beautiful sunset! The warm glow of the sun provides a spectacle of colors as light passes through the earth’s atmosphere. Underscoring this breathtaking event is an unusual phenomenon called a green flash. If the conditions are right, this burst of emerald light occurs at the last moment of a sunset. A rarer event called a blue flash is said to be even more beautiful.

What causes these colorful flashes? Why do they last for only an instant? And why are they so rare? To answer these questions, we must first have a basic understanding of the interaction between sunlight and the earth’s atmosphere.

Sunlight streaming down to the earth contains all the colors of the rainbow. When this light strikes the earth’s atmosphere, the atmosphere acts much like a giant prism and scatters, or diffuses, the light. The extent to which a light wave is diffused, however, depends on its wavelength.

Blue light waves have a shorter wavelength and are widely scattered throughout the atmosphere. That is why the sky appears blue when the sun is well above the horizon on a clear day. But when the sun is near the horizon—such as at sunset—the sunlight must travel through more of the atmosphere to reach our eyes. As a result, the highly scattered blue light does not reach us. On the other hand, the longer waves, such as red, can more easily penetrate the dense atmosphere. This gives the sunset its familiar red or orange color.a

Under certain conditions, though, a green flash or a blue flash can be seen at sunset. How do these occur? As the last edge of the sun dips below the horizon, the sun’s light splits into a spectrum like a rainbow. Red light appears on the bottom of the spectrum, and blue light is on the top. As the sun continues to descend, the red portion of the spectrum falls below the horizon and the blue portion is usually scattered by the atmosphere. It is at this instant that the last bit of visible light can flash green. But why green? Because green is the other primary color in light.

When the sky is heavily polluted, the green flash is seldom seen, and the blue flash occurs only when the atmosphere is exceptionally clear and enough blue light pierces the sky to cause a brilliant flash to appear.

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Sunset: ©Pekka Parviainen/SPL/Photo Researchers