Why is the sky blue? And the sunset red?

Believe it or not, I get asked this question all the time. While most people assume they know the answer, the truth is, we often don’t remember what we learned as a kid (or we never learned it). A profound question indeed, and not as simple as you might expect. Well, let’s get to it!

The reason the sky is blue has to do with a concept called Rayleigh Scattering. Let’s say we have a gas, and we shine light on it. The light will hit the gas molecules, and get absorbed. Then, the gas molecules will emit the light back out at the same wavelength (color!), but in a new direction. Interestingly enough, blue light (shorter wavelength) gets absorbed more than red light (longer wavelength). This scattering of incoming light is Rayleigh Scattering!

Have you figured it out yet? Now imagine the sun, shining light towards the Earth. Remember, this is white light, which means it contains light of all visible colors at once! As the white light from the Sun hits the Earth’s atmosphere (a gas!), the molecules absorb and emit the blue light, and scatter it all around. So when you look up at the sky, you’re seeing all of this scattered blue light. But why is the sky paler near the horizon? At the horizon, light has to pass through more of the atmosphere, and in the same way blue light is scattered towards you, it ends up getting scattered away again, and so the other wavelengths are the ones that get through, since they are less affected by scattering. Here’s a nifty diagram I found at http://www.sciencemadesimple.com/sky_blue.html :

   (As you can see, the blue lines represent the paths of blue light beams)

This horizon effect is the same one that causes the sunset! As the sun is setting on the horizon, the light once again has to pass through more of the atmosphere in order to reach you, and so the blue light gets scattered away, and the redder light is what reaches you. But why are some sunsets more spectacular than others? In addition to the atmosphere itself, dust particles and water particles in the atmosphere can contribute even more to the scattering of shorter wavelength light beams, and so you get a dazzling display of red, orange, and yellow hues. The more particles in the atmosphere, the more variety you can end up with–and so the sky will look like a majestic painting.

So why is this cool? Indeed, it’s nice to know why the sky can be so beautiful, but it’s not the only reason Rayleigh scattering can be cool. A friend of mine recently asked me why the Moon was so red, even though it was not a lunar eclipse (that’s a whole different cool phenomenon). When you see the moon, you’re really seeing sunlight that’s reflected off of the moon’s surface. When the Moon is near the horizon, or if there are lots of these particles in the air, the light reflected off the Moon gets scattered the same way sunlight is scattered during the day. This causes the Moon to appear the color of the sunset! For a fantastic picture, as well as fun facts, visit http://apod.nasa.gov/apod/ap050922.html

Hope you enjoyed my first entry! For more information, I’d advise checking out the two sites I mentioned in more detail. Of course, a google search should bring you to some great websites as well. Stay tuned for more cool science!

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2 Responses to Why is the sky blue? And the sunset red?

  1. Karen says:

    but why is the sky GREEN sometimes (like during that pseudo-tornado we had a few weeks ago)?

  2. Great question! To give you an answer, I’ll quote from an article in Scientific American:

    “Researchers remain undecided about the exact mechanisms that cause the sky to appear green in certain thunderstorms, but most point to the liquid water content in the air. The moisture particles are so small that they can bend the light and alter its appearance to the observer. These water droplets absorb red light, making the scattered light appear blue. If this blue scattered light is set against an environment heavy in red light—during sunset for instance—and a dark gray thunderstorm cloud, the net effect can make the sky appear faintly green. In fact, green thunderstorms are most commonly reported in the late afternoon and evening, according to Beasley.”
    Source: http://www.scientificamerican.com/article.cfm?id=fact-or-fiction-if-sky-is-green-run-for-cover-tornado-is-coming

    It’s the same effect from normal scattering, but during a severe thunderstorm, you have lots of those water droplets and particles in the air, which further absorbs red and reflects blue. Usually green happens at night, because the scattered blue light blends with the dominant red colors, and so you end up with a greenish tinge. So it’s not the tornado that does it, but rather the thunderstorm itself (and water in the air because of it).

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