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Question of the Day
Question:
"Dr. Hal, why is the sky blue?"Answer:
I get this one a lot; it usually comes with, "why is the grass green?" That question concerns plant physiology as well as the properties of light, making the former query simpler to answer. Now here's something to think about. At night the sky is black, with the moon, planets and stars all points of light surrounded by darkness. Why, then, is the sky not also black by day, with the sun just another light spot on a dark field? Instead, as we know, it's a bright blue and the stars vanish. This isn't (or need not be) such a frivolous question. Edgar Allan Poe determined that the universe was infinite by the fact that it gets dark at night-- in a limited cosmos, light from the stars would provide full illumination. Anyway, our Sun is the brightest thing in the sky, you'll agree. An extremely bright source of light. Then, our atmosphere, remember, contains nitrogen and oxygen; atoms of these gases effect the solar rays passing through them. Now I'll probably get Pete Goldie on my case for misstating the facts, but there's something known as "Rayleigh scattering" that causes light to scatter when it passes through particles that have a diameter one tenth that of the wavelength (color) of the light. Recall that sunlight is made up of all different colors, but because of the elements in the atmosphere, the color blue is scattered much more efficiently than the rest. When you look at the sky on a reasonably clear day, the Sun appears as a bright disk. The blueness you see everywhere else is all of the atoms in the atmosphere scattering blue light toward you, but not dispensing red light, yellow light green light or any of the other colors nearly as well...
Askdrhal.com by
Jascha Ephraim,
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