- Have you ever wondered
why the sky is blue?
Why is it that the sun, which is actually
white in color, looks yellow to us?
Why does the sunlight
usually look yellow to us?
And why is it that the sunrise
or the sunsets are usually red in color?
Well, the short answer for this
is because the molecules
of our atmosphere,
like the nitrogen molecules
or the oxygen molecules,
tend to scatter blue light more than red.
So let's explore this in
a little bit more detail.
We've talked about scattering
of light in previous videos.
Basically, when light
hits a tiny particle,
it reflects light in all the direction,
and that's what we call a scattering.
When this scattered light enters our eyes,
we see that particle glowing the
same color that it ends up scattering.
So if it scatters, let's say yellow light,
then the particle will glow yellow to us.
Now, the important thing
is that the molecules
of our atmosphere don't scatter
all the colors of light equally.
Now do you understand why
that's a little complicated?
Because the physics of scattering
is a little bit complicated.
But in short, what's happening is that
you see, our white light is actually
made up of seven colors: "VIBGYOR,"
the seven colors of the rainbow.
The reason we even see
these different colors,
is actually because light is a wave,
or it can be thought of as a
wave, like a wave on a string.
And then if you were
to look at these waves,
then it turns out that the
shorter wavelength then to
hit our eyes, we perceive that as violet.
The longer wavelength then to
hit our eyes, we tend
to perceive it as red.
So as the wavelength becomes longer,
the color changes from
violet towards the red.
And when we do the physics of scattering,
it turns out that when we're dealing with
particles of the atmosphere which are
much smaller than the wavelength of light,
they always tend to scatter the
shorter wavelength more compared
to the longer wavelength.
Okay? We're now going
to see why that happens.
As I said, that's a little difficult,
well that's not little, that's
actually pretty complicated.
But it turns out that
the shorter wavelength,
which we see as violet, so
this shorter wavelength,
let's write that down somewhere over here.
So this shorter wavelength
scatters the most,
and the longer red wavelength
scatters the least.
Therefore, if white light were to come
and hit one of these
atmospheric molecules,
then they would tend to scatter blue light
much more than the red light
because blue has a shorter wavelength,
somewhere over here, compared to the red.
Now I'm pretty sure we might be wondering,
"Why blue? Why not the violet itself?"
Because that is an even
shorter wavelength, isn't it?
Well, it turns out that
the sun doesn't produce
enough of violet in the first place.
So there isn't much violet light
in the incoming sunlight, and therefore
there won't be much violet
in the scattered light.
Another reason why we don't see violet
is because our eyes turn out to be
not so sensitive to violet at all.
Combined with these results,
we don't tend to see violet,
but we tend to see this indigo blue,
which together we tend to
usually call it as blue.
So that's the blue light that
gets scattered the most, and therefore
the atmospheric molecules tend to glow,
they tend to glow blue in color,
when seen from any direction.
Now we are ready to answer
all of our questions.
So let's get rid of these pictures.
First, let's look at what we would
have seen if there wasn't an atmosphere.
So let's say it's the daytime,
that means the sun is right above us.
Something like afternoon.
Then, the rays of light
would be coming straight down like this.
Now if we were to look in
the direction of the sun,
in this direction, then we
would see the white sun.
But if we were to look
in any other direction,
then there's no light coming towards us
from any of the direction because
all of the light is
just falling downwards.
So we would see nothing. Therefore,
all you would be seeing
without an atmosphere,
is the white sun and everything
else would just look dark.
And you know what? Let's perform
an experiment side by side.
All you would need is a flashlight,
a tank of water, and some milk.
When we add milk to this water,
the milk particles are going to
represent our atmosphere.
So if you look at the flashlight directly,
without any milk particles in between,
then all we see is the white light.
And look at the sky, the
whole thing has become dark.
Exactly like what we would see
over here without an atmosphere.
But now let's bring in the atmosphere.
When we bring in the atmosphere,
the sunlight strikes all this
atmospheric particles
and like we discussed,
they'll end up scattering
blue light the most.
They do scatter all the other colors,
but blue light gets scattered the most.
And as a result, almost all these
atmospheric particles
will end up glowing blue.
So now, in whichever direction we look,
all we would see is the blue light.
And now in our experiment, if we bring
the tank of water and start adding milk,
the milk particles are going to
mimic our atmosphere. Just like our
atmospheric particles, they are scattering
blue light the most. And as a result
the whole tank is glowing blue.
Because everywhere, the milk particles
are mostly throwing blue light towards us.
Beautiful, isn't it? But did you notice,
as the sky became blue,
the sun turned yellow?
Why did it turn yellow?
Well, if you come back over here,
the initial incoming rays are white.
But once it hit the atmospheric particles,
they start scattering blue light.
So from the incoming light,
blue got scattered away.
So if you look at the colors that
are remaining now in this incoming light,
violet was never there in
the first place, not much,
indigo blue got scattered away,
so now the incoming rays only contain
green, yellow, orange, and red.
Even these are being scattered,
but not as much as blue.
So when we combine these colors together,
we get that yellowish glow.
And that's why the sunlight now,
once it has entered the atmosphere,
only retained that yellowish glow.
Because most of the blue
has been scattered away.
That's why when we look in
the direction of the sun,
the sun starts looking yellow to us.
The same thing is happening
in our experiment as well.
All right. Finally, what happens
during the sunset or the sunrise?
Well, the effect or the concept
is pretty much the same.
The only difference now is that
the sun is near the horizon.
So let's say the sun
is somewhere over here.
Then the rays of sunlight makes its way
through the atmosphere reaching us.
Notice now it's passing
through a much longer
part of an atmosphere compared to before.
And so by the time this
light reaches our eyes,
not only is the blue light
being scattered away,
but pretty much the green
and yellow is also gone
because it's hitting so many
more atmospheric molecules.
So the only color that survives
are the long wavelengths which get
scattered the least,
that is orangeish-red.
Therefore, by the time
this light reaches us,
it's going to look pretty
much orangeish-red.
We can see the same thing
in our experiment as well.
If we look from the top,
we can see the light
is passing through the short part
of the tank, only the short
part of the atmosphere.
So what we can do, is
we can take that tank
and we can turn it so that we make
the light pass through the larger part,
the longer part of the tank.
Now if we see it from the front,
notice the sun turns red,
or at least reddish-orange.
Beautiful, isn't it?
Also notice that the light that
is reaching us is pretty dull,
we can hardly see the flashlight now.
That's because most of the light
has been scattered away from us.
That's what makes the sunrise
and the sunset so beautiful,
because we can see it directly
without hurting our eyes.
To quickly summarize,
the reason we see the
blue sky and the red sunset
is because when light,
or white light hits these
atmospheric particles,
they scatter blue light more
compared to all the other colors.
One last thing I want to
talk about before we wind up,
is why are the clouds white in color?
What I mean is that clouds are
made up of tiny drops of water.
So when sunlight hits
those drops of water,
shouldn't they also scatter
blue light in all the direction?
That means shouldn't the
clouds appear blue in color?
Well, it turns out that
only those particles
whose size is smaller than
the wavelength of light,
only those particles can
scatter blue light more
compared to red; or shorter wavelengths
more compared to the longer ones.
They're oxygen, nitrogen molecules.
Now the molecules of the atmosphere
and the milk particles
that we find in water,
pretty much fall into that category.
But if the particles become much larger
than the wavelength of light,
in such cases it turns out that
they will scatter all the colors equally.
So drops of water are actually,
which are found in clouds,
are much larger than the
wavelengths of light.
So when white light falls on them,
they scatter all the colors equally.
As a result they end up
scattering just white light.
It is for that reason we
will see clouds to be white.
Even fog or mist also
fall into that category.
Even they appear pretty white to us.
