(bright playful music)
- Hey, I'm Ben Pearce, I'm a PhD student
in astrophysics and astrobiology
at McMaster University,
and I'm answering why is the sky blue.
Well, the sky's not always blue.
You may recognize that at
night, the sky's actually black.
And it's not until we
get that daytime sky,
when the sun's actually out,
that we get to see that blue color.
And this is our first hint that
the sun plays a large role
in the cause of the blue sky.
But what is blue, anyway?
What are any of the colors?
Well, really, they're just
different wavelengths of light.
Red is the longest wavelength
of light that humans can see.
And blue is some of the
shortest wavelengths.
It's actually violet light that is
the shortest wavelength
that humans can see.
Now the sun generates all
different kinds of light,
all the colors of the rainbow,
and some colors that we can't even see,
things like ultraviolet
light and infrared light.
Now when that light makes
its way from the sun
to the top of the Earth's atmosphere,
and enters into the atmosphere,
it's gonna scatter off tiny
particles in the atmosphere,
things like nitrogen and
oxygen that we breathe.
Now it turns out that blue light
scatters about 16 times more
frequently than red light.
So if you're looking at
any random part of the sky,
you're going to see about 16
times more indirect blue light
than you are indirect red light.
Now, it turns out that the
shorter you get in wavelength,
the more frequently you scatter.
So violet light, you would
think, would scatter the most.
And in fact, it does.
So why don't we see a
beautiful violet-colored light?
Why is it blue?
Well, the answer's twofold.
As it turns out, the
sun actually generates
a little bit less violet
light than it does blue light,
about two or three times less.
And the second aspect has
to do with the human eye.
There are three different kinds
of cones in the human eye,
one sensitive for red light,
one sensitive for green light,
and the last is sensitive for blue light.
Now that cone that's
sensitive for blue light
also is responsible for giving us
these violet colors that we see,
but it's about two or
three times less sensitive
for violet light than for blue light,
and that's why we see the blue sky.
Now it's important to remember that
not all species see a blue sky.
In fact, there are many kinds of spiders
that are sensitive to green
light and ultraviolet light,
and types of snakes and bees that
are sensitive to infrared light.
So although we see the blue sky,
the blue sky is not blue for everyone.
