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  • Hey, everybody.

  • I'm Ben, and here's today's question.

  • Why is the sky blue?

  • This is one of those questions that four-year-olds ask all the time, and it's something interesting to think about.

  • So let's start here. When you look at the sky at night it's black, and there are stars, and there's the moon,

  • and they form these points of light on that black background.

  • So why is it that during the day the sky doesn't remain black with the sun acting like just another point of light?

  • I mean, that's exactly what happens on the moon.

  • So why on Earth-- huh, on Earth--does the daytime sky turn a bright blue,

  • and all the stars except for the sun seem to disappear during the day?

  • There's a couple things we have to recognize.

  • First, the sun is an extremely bright point of light. It's also much closer to Earth.

  • The second thing to recognize is that the atoms and nitrogen

  • and oxygen in the atmosphere havean effect on the sunlight that passes through them.

  • Also, there's scattering that causes light to scatter throughout the sky when it passes through particles

  • that have a diameter that's 1/10 that of the wavelength or the color of the light.

  • Now, sunlight is made up of all different colors of light.

  • But because of the elements in our atmosphere, the color blue is scattered much more efficiently than all of the other colors.

  • So when you and I look at the sky on a clear day, we can see the sun as a bright disk.

  • And that blueness that we see everywhere else is all of the atoms in the atmosphere scattering blue light toward us.

  • That's because red light, yellow light, green light,

  • all of the other colors are not scattered nearly as well and we see the sky as blue as a result.

  • And that blue color is so bright that it completely blots out all the other stars because they're so dim in comparison.

  • So that's it. That is why the sky is blue.

  • Thanks so much for watching.

  • We hope you enjoyed this episode.

  • And as always, you know what I'll say at this point.

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  • These videos are cool.

  • These videos are cool.

  • You know, I feel safe saying that all three of these videos

  • are cool.

Hey, everybody.

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Why Is The Sky Blue?

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    Ray Du posted on 2015/01/22
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