I’ve genuinely never never understood why the sky is blue. Or why everything isn’t
blue? If there’s so much blue light coming in or whatever. I don’t know.
Here’s what I learned.
Imagine being in space with just you and a light source. It’s giving off light all
around you. This is what that would look like.
It’s looks dark over here and over here, because there’s no light coming from over
there or over there. You can only see something, when there’s light coming into your eye
from that direction. So here, we just see the light source.
On earth, you’ve got light coming in from the sun like this. Some of that light hits
your eyes. Which is why you can see the sun. Most of that light doesn’t hit your eyes
and you can’t see it. But some of that light hits clouds and trees and stuff, the light
then scatters, some of it scatters in the direction of your eye balls. Which is why
you can see clouds and trees and stuff. And if that’s all that happened this is
what that would look like.
You can see the clouds and trees and stuff, you can see the sun. Except not in this photo.
And the sky would be dark.
But instead we’ve got blue light coming from empty sky.
It’s phenomena where sun light will scatter off the nitrogen and oxygen molecules in the
atmosphere. The molecules are bouncing the photons all
over the place. How often they scatter depends on the wavelength.
The shorter wavelengths tend to scatter more.
So out of the sunlight which is a mix of a bunch of wavelengths of light.
We’ve got longer wavelengths scattering a bit. And we’ve got shorter wavelength
scattering relatively more. Which makes more of the blue light hit our eyes from over there
so it looks blue over there. In a way you’re just seeing the air.
Light bounces off of air molecules and then heads towards your eyes.
Kind of like how you can see that mountain because light bounces off of it and then heads
towards your eyes.
If the air scattered all wavelengths equally, instead of scattering shorter wavelengths
more, it would look white or grey instead of blue because all the wavelengths together
make white.
Even things off in the distance will become blue. The mountain is so far away that there’s
enough gas molecules between us and it for us to see the combined scattered light.
They call this Rayleigh Scattering. Scattering because it scatters in all directions. And
Rayleigh, because people have names.
But this effect that causes blue light to come in from the sky, doesn’t actually make
everything blue. Because the atmosphere isn’t like a blue
filter, only letting blue light pass through it.
And the atmosphere isn’t absorbing all the wavelengths and then glowing blue, sort of
like a neon light or something. And the atmosphere isn’t itself blue, like
a puff of blue smoke, again, absorbing all the other wavelengths.
OK, all the wavelengths are still coming in. And that light is the majority of the light.
I mean maybe because we can’t see that light coming in, but we see this blue from everywhere
That it kind of seems like everything should be more blue.
But that blue is just the relatively small amount that’s Rayleigh scattering about
The full spectrum is still striking objects and allowing for a full spectrum of light
to scatter. So we can have blue light coming in from the sky without it making everything
blue.
But wouldn’t… Shouldn’t things be more blue because of
all the extra blue that’s coming in from every angle in the sky?
Even though it’s not that much blue, shouldn’t it accumulate?
No. This drawing might be a little bit confusing, at least it confuses me.
Bad drawing bad drawing. Instead lets just look at the blue light.
If it scattered like this, the light is coming in at different angles. But the amount of
light that scatters is the same amount that would have been in direct light.
On average no spot is going to get more or less light and nothing is going to be any
more or less blue.
But if it scattered like this in all directions, like it does on Earth, then we’re going
to lose some of that blue light to space. It’s why you can see the sky in space.
So on Earth we’re only getting less blue light than we would if there wasn’t any
scattering. And everything should be less blue than they
would have been above the atmosphere. But the effect seems to be small. Looking
at the space people things look about the same. The light doesn’t look more blue than
the light that reaches us.
Unless if we compare it to sunrise and sunset. During the day light doesn’t have to travel
through that much air, and we’re not losing a whole lot of blue.
But when sunlight has to travel through more atmosphere and thicker atmosphere close to
the ground. So much of the shorter wavelengths have scattered
away, that the light that reaches us becomes orange or golden.
They call it golden hour.
And when the sun is even lower in the sky, the straighter shooting longer wavelengths
are mostly shooting straight off into space. But the shorter wavelengths are still bouncing
around. Some of that light hits objects which tends
to make everything blue. They call it blue hour.
Here’s them side by side. And here’s a timelapse with inspiring music.
Timelapses are great. Normal hour, golden hour, normal hour, golden
hour, blue hour, blue hour. Ree errr ree errr.
They’re called hours but they don’t necessarily last an hour, its like “The Hundred Years
War”.
Here’s both together in the same shot The straight shooting longer wavelengths make
up the majority of direct light, and the blue light is going on an adventure and hitting
spots in shadow to direct light.
Why does the sky get white near the horizon? Two reasons.
First, rayleigh scattering. As we look down more we’re looking through a longer and
denser section of atmosphere. Which allows for more air molecules for the longer wavelengths
to accumulate. Remember all the wavelengths scatter, the
short ones scatter more so scattered light looks blue. But only when you’re close to
it. Because the blue light from far away, doesn’t
really make it this far. Those longer wavelengths scatter less, but
they shoot straighter when they do. So when we look through more air molecules we start
to get a greater proportion of the longer wavelengths.
Looking at the horizon, we get short wavelengths from air close to us, and an accumulation
of the longer wavelengths from the whole thing, we’re getting more of all the wavelengths
so it looks more white and we’re getting more total light so it looks brighter.
The mountains right below the horizon are still blue because they’re not far away
enough for the longer wavelengths to accumulate. The second reason the sky is lighter near
the horizon is from a different kind of scattering. Rayleigh scattering is when light scatters
off of really tiny molecules like oxygen and nitrogen gas.
But light can also scatter off of bigger stuff like microscopic water molecules, little bits
of dust, pollution, pollen. And these things tend to scatter all the wavelengths
equally. Which means sunlight that scatters off of
these particles will look white. The closer to the horizon you look the more
of these particles you’re looking through. So over long distances that white light accumulates
and it drowns out the blue. It’s kind of like looking through an incredible
sparse fog.
When light bounces off these kinds of particles they call this Mie scattering.
Scattering, because it scatters. And Mie because this guy had a name.
Rayleigh scattering and Mie scattering would also explain the colours of the sky at sunrise
and sunset. You’ve got that golden light coming in and
its bouncing off of air molecules or the little bits of stuff floating in the air.
Although pollution and these other molecules can scatter away so much of that light that
it becomes dull. Kind of like how Rayleigh scattering scatters away the blue light.
Often the most beautiful sunsets are when the is clean and clear allowing a strong light
to come in so it just can bounce off the clouds that are near.
The last question we’re going to look at, is maybe the first questions you should have
asked. Can you trust This Place? Can you trust me,
this guy?
Just what kind of person is this man? Can you rely on anything he says?
You know, maybe he understands how biased we are, that we have to help each other through
and aim for ultimate humility. But secretly he keeps the Youtube plaque above
his bed. So the ladies know what’s up.
Maybe he knows how to navigate the scientific literature and is grateful for an education
that gave him that skill. But sometimes can’t be bothered and just
gets the information from a banana. He doesn’t correct his mistakes.
He might encourage you to examine the science for yourself. But only because he suspects
you won’t and hopes you will think he’s reliable for saying that.
Can you trust this man? Inspiring music
I have a lot to learn. But maybe one day I’ll be… okay you know good enough.
Can you trust that I will try to be a moderate and disinterested voice?
Can you trust that I will find an acceptable balance between depth and understandability?
Can you trust that my goal is to get as close to the truth as possible in this absurd and
expansive place?
I don’t know but this episode is brought to you by the Got Brush toothbrush.
It’s got bristles. It’s got 50% more bristles than the next leading brand.
1/5 dentists said f*** me is it for a beaver? It could strip the rust off a hubcap.
I’m required to say: the first 10 people to use the promo code, for free, will get
brush. *Sigh. Oh my god.