Today I will explain why the sky is blue
and why sunsets are red unless you're on
Mars in which case the sky is red and
sunsets are blue.
First, you should know that white
sunlight is made up of all the colors of
the rainbow.
In fact, rainbows are formed when rain
splits white light into all the
different colors and when leprechauns
hide pots of gold. So why is the sky blue?
To answer this question I'm going to
have to get really really microscopical!
Here we have a green light wave. If I
lengthen the light wave, we get red light.
If I shorten the light wave, we get blue light.
To my right, I have a box full of
air molecules. If I shine a blue light at
the box. The light waves scatter in every
direction. But if I shine red light at
the box, the light doesn't scatter as
much. This is called Rayleigh scattering.
Now if we have a larger particle such as
a water droplet found in clouds, then we
get a different kind of scattering
called Mie scattering.
Mie scattering scatters all wavelengths
of light and it's why clouds are white.
Mie scattering causes light to scatter in
the presence of large particles found in
dust and clouds. Rayleigh scattering
causes blue light to scatter in the
atmosphere. When sunlight hits the
Earth's atmosphere,
the blue component scatters in every
direction and this is why the sky is
blue. During the day, not that much
scattering is happening because sunlight
is traveling through the Earth's
atmosphere relatively quickly. But at
sunset, the sunlight has a much longer
path through the atmosphere and so the
blue light scatters
way, the green light scatters away, the
yellow light scatters away. 99% of the
light is gone. Only the red remains and
this is why sunsets are red. You can even
create your very own personal sunset at
home. All you need to do is get a
flashlight,
fill up a tank of water and add a little
milk. The milk contains small particles
which scatter the blue light. And now we
have our very own blue sky. With the blue
light scattered away,
what remains at the end of the tank is
just the yellow and red light.
But something very different is
happening on Mars. Martian skies are red.
Mars doesn't have much air in its
atmosphere but what it does have is dust
particles. These dust particles don't
cause Rayleigh scattering they cause Mie
scattering. And the dust particles are
also colored red just like the surface
of Mars. Mars is red because it contains
iron. Iron when it rusts turns red. This
is the same reason why blood is red
because it contains lots of iron. One
person who really likes blood is Mars,
the Roman god of war. The planet Mars is
named after the god Mars because it's
his favorite color. On Mars sunsets are
actually blue. This is an effect caused
by Mie scattering. With Rayleigh
scattering blue light scatters equally
in all directions, but with Mie scattering
the light tends to scatter more in the
forward direction and this forward
scattering effect happens more for blue
light than red light. This causes a large
blue halo to form around the Sun as its
setting on Mars. The same effect can
happen on earth. In 1883, a giant volcanic
eruption on Krakatau spewed enough ash
and dust into the atmosphere that the
moon turned blue for several months. So
the same blue sunset effect can be seen
on earth, but only once in a blue moon.
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