Podcaster: Richard Drumm
Title: Why is the sky blue?
Organization: Astrosphere New Media
Link : http://astrosphere.org
Description: Now this is the archetype, the typical question that a child asks an adult. Now I can’t remember if my daughters asked it, but some kids must have asked it at some point, so listen carefully to this podcast and you’ll be ready to explain the sky’s color should the occasion ever arise.
Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.
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Transcript
Welcome to the Q&A series of the 365 Days of Astronomy. If you have a question which you’d like us to answer, visit our G+ or facebook pages and fire away!
Today’s question:
Why is the sky blue?
Now this is the archetype, the typical question that a child asks an adult. Now I can’t remember if my daughters asked it, but some kids must have asked it at some point, so listen carefully to this podcast and you’ll be ready to explain the sky’s color should the occasion ever arise.
OK.
First a little bit about sunlight. There is the invisible infrared light, the visible Roy G. Biv colors; you know, the red, orange, green, blue, indigo & violet, and then there’s the invisible again, the ultra violet. There’s even radio & microwaves down past the infrared and x rays up past the ultraviolet for that matter, they’re all streaming off the Sun. This progression is from longer wavelengths to shorter, which represents lower frequencies to higher frequencies and lower energies to higher energies.
Anyway, the sunlight when it arrives here at the Earth is a mix of all these wavelengths or colors. The light passes through the upper atmosphere then the lower atmosphere and then works its way on down through to the ground. Now as the light passes through the air the atoms & molecules of gas and dust in it have a chance to mess with it.
A process called Rayleigh scattering basically separates out the bluish light (and a good bit of the green) from the red light and that causes the blue color. This is also why sunsets are red so if a kid asks you why the sunset’s red, well, you’ll be ready for that question too.
Now this is a different process than what happens with a piece of blue stained glass. There you have red light being stopped dead in its tracks, being absorbed & blue light going through undisturbed. Rayleigh scattering has just the opposite, the blue getting disturbed and red passing through. The blue light isn’t absorbed or re-emitted or anything like that with Rayleigh scattering, it’s merely bent onto a new path. It keeps its energy (which is the same as its color) and it only gets a new direction of travel. Physicists call this an elastic process. This has nothing to do with rubber bands.
You can think of the air as a weird kinda pachinko pinball machine where the large reddish balls pass through the maze of pins without hitting anything and the small bluish balls get bounced around randomly. If you don’t know what a pachinko machine is, Google it. It’s a vertical pinball machine without the flippers and is sort of like a Japanese slot machine.
So here’s how it works here in Virginia. As the Sun starts to set in the West, the sunlight is passing through the upper atmosphere over Kentucky, then the lower atmosphere over West Virginia, then it reaches our eyes here in Virginia. Now out West it’s not as close to sundown and the blue light is bounced out of the sunbeam, turning the sky there blue, and the red part continues Eastward to Virginia where it lights up the sunset with all that red light.
The folks out West see a blue sky and we see a red sunset.
This Rayleigh scattering is also why people call the Sun yellow. With the blue light and some of the green somewhat reduced, the white light of the Sun takes on a slight yellowish tint. You might think that this would cause us to see snow as yellow and make it impossible not to eat yellow snow, but all that blue light coming from the blue sky, well that mixes back in with the yellowish & reddish light and makes the snow look white again. So we can tell where the dog has, uh, done his business!
End of podcast:
365 Days of Astronomy
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The 365 Days of Astronomy Podcast is produced by Astrosphere New Media. Audio post-production by Richard Drumm. Bandwidth donated by libsyn.com and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. In the new year the 365 Days of Astronomy project will be something different than before….Until then…goodbye