Why is the sky blue?
San Rafael, Calif.
We see the "green flash" occasionally as the sun sets behind the Pacific Ocean. Can you explain this phenomenon?
La Jolla, Calif.
Journey if you will to the 17th century, when a bright Briton named Sir Isaac Newton discovered that when light passes through one medium (air) into a denser medium (glass), it bends. The process is called refraction - the same phenomenon that makes a stick appear to bend when you insert it into a pool of clear water.
Using a prism and the principle of refraction, Newton bent "white" sunlight, breaking it into its component colors - red, orange, yellow, green, blue, indigo, and violet - all of which have different frequencies or wavelengths, the same way your favorite radio stations on the FM dial do. Wavelengths for blue light are shorter than for red.
But a prism is not the only way to get color out of white light. It turns out that air molecules and other tiny particles in the atmosphere do a good job of absorbing and re-emitting blue and violet light in random directions, while leaving the other colors relatively unaffected. This scattering makes it look as if the blue is coming from everywhere in the sky at once.
As atmospheric particles grow in size, they grow less selective about the wavelengths of light they scatter. By the time particles reach the size of water droplets in clouds, they scatter light at all wavelengths, yielding white clouds.
So much for blue. What about the warmer colors of sunrise and sunset? The sun is lower on the horizon, and its light is traveling farther through the atmosphere. By the time the light reaches a viewer's eye, the atmosphere has scattered most of the blue light, leaving colors at the red end of the spectrum.
Between blue and red lies the color green, which is where the "green flash" comes in. The green flash represents the instant at which sunlight hits the atmosphere at just the proper angle from the viewer's perspective to break out (refract, as in Newton's prism) a bit of green. In order to see green flashes, a viewer needs an unobstructed horizon (like the Pacific Ocean) and extremely clean air so there is little to scatter the green light. The flashes occur either just before the first bit of sun peeks over the horizon at dawn or after the last fragment vanishes at sunset.
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