[MUSIC PLAYING]
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 have
an 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.
And 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.
You know, I feel safe saying that all three of these videos
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