My Chinese name is Tu Zhongfang.
My English name, you can call me Jason.
Why is our sky blue?
Hello, my name is Samantha Tushaus and
I am a researcher at the
Space Science and Engineering Center at
the University of Wisconsin-Madison.
And today I'm going to tell you why
the sky is blue.
Light from the sun comes in many different
colors,
including colors that we see and some
colors that we don't see.
Just like sound waves or waves on the
water, light travels in waves as well
and each color of light has a different
kind of wave.
There are short wavelength waves, which
means that the tops of the waves
are close together.
And long wavelength waves where the tops
of the waves are farther apart.
Blue light has a shorter wavelength
than red light.
So light from the sun will travel in a
straight line until it encounters
something like the earth's atmosphere,
which is made up of billions of tiny
molecules of mostly nitrogen and oxygen.
When the sunlight encounters the 
atmosphere,
the molecules in the atmosphere like to
scatter shorter wavelengths more than
they like to scatter long wavelengths.
So the molecules in the atmosphere scatter
the blue light much more than they
scatter red light.
That's why when we look up at the sky,
we see blue light.
That's all the blue light that's been 
scattered by the
oxygen and nitrogen in our atmosphere.
Now you might be wondering, purple
actually has a shorter wavelength
than blue does, so why isn't
the sky purple?
It turns out that a lot of the purple
is scattered out really high in the
atmosphere,
so there isn't much left to reach
our eyes, so we see blue.
But more importantly, our eyes don't
recognize purple
as well as they do blue.
And finally, you might also be wondering
why sunsets and sunrises
sometimes look yellow, orange or red
instead of blue.
It turns out that the sun, at sunrise
and sunset is much farther away
from us at the horizons instead of
overhead.
There's a lot more atmosphere that the
sun's light has to travel through to
get to our eyes.
Along the way most of the blue 
light is scattered so much that
there's hardly any left by the time the 
sunlight gets to our eyes.
All that's leftover is the red, yellow
and orange light.
And that's why we see the red, yellow and
orange at sunrise and sunset
as opposed to the blue sky we see 
when we look up during the day.
