One of the most common misconceptions that
I believe before was that our Sun is a “yellow
star”.
That is not true!
In fact, the sun is a white star!
You can see it from the photographs taken
from the outer space.
But why does the sun looks yellowish then?
Well, the reason for that is the same reason
as to why is the sky blue.
So why is the sky blue?
Many philosophers and scientists in the past
had also asked the same question, from Aristotle,
Da Vinci, Newton, they all wondered what makes
the sky blue, and it took us a very long time
to figure it out.
The sky is blue, because the blue light from
the white light of the sun, is being scattered
by the air molecules in our atmosphere and
making our sky blue!
But how can a white light produce a blue light
in the first place?
Well, you see, the white light from our sun
is actually a combination of different colors
of light, including red, orange, yellow, green,
blue and violet light!
We know this because if we let a white light
to pass through a glass prism, we can see
the separation of these different colors of
the light which is what we now call as the
“spectrum of light".
Sometimes, even tiny water droplets suspended
in the air (especially after a heavy rainfall)
can also act as tiny prisms, separating the
different colors of the light from the sun,
and we call this phenomenon as a “rainbow”.
Now, as these different colors of light from
the sun enter our planet, they interact with
the air molecules in our atmosphere causing
each color of light to be scattered in different
directions, but the blue light is the most
scattered color among them all, and that is
why our sky is blue!
This light scattering phenomenon is called
the “Rayleigh scattering”, named after
this guy who discovers it.
But what’s up with the blue color?
Why is it the most scattered color?
Why not red, or green?
Well, it has something to do with its wavelength.
You see, we can imagine light in two ways:
we can imagine it as a particle (which we
call it “photons”) or we can imagine it
as a wave, like the waves in the surface of
water.
Light behaves like a particle and a wave at
the same time.
And if you’re wondering how can this be
possible, well, that’s what our scientists
are still trying to figure out.
Now, if we choose to imagine light as a wave,
then, therefore, light must have a wavelength!
Wavelength is just the distance over which
the shape of the wave repeats.
You can measure the wavelength of a wave like
this,
or this,
or this.
Light can have a wavelength from as long as
100 kilometers, to as short as 1 picometer.
But in spite of this wide range of wavelengths
that light can have, the human eyes are sensitive
only to lights having a wavelength from 400
nanometers to 700 nanometers.
The light within this range is called the
“visible light” since these are the only
lights that are visible to human eyes.
Within the visible light, the longest wavelength
of light that our eyes can see is around 700
nanometers, which we see as color red!
On the other end, the shortest wavelength
of light that our eyes can see is at around
400 nanometers, which we see as color violet!
And all lights within this range are the different
colors of light that we see in the rainbow.
Here’s the table of the different colors
of light, and its corresponding wavelengths.
Notice here that the blue light has a relatively
short wavelength compared to the other colors.
Now, Rayleigh had proved, through this mathematical
expression that he was able to derive, that
the shorter the wavelength of the light, like
the blue light, the higher the intensity of
its scattering.
Although this expression might seem too complicated,
but our main takeaway from here is that a
shorter wavelength of light (which is denoted
by lambda) means a higher intensity of scattering
(which is denoted as slanted capital "I"),
and a longer wavelength of light means a lower
intensity of scattering.
This explains why blue is the color of the
sky and not just any other color!
What’s great about this is that Rayleigh’s
discovery does not only explains the blueness
of the sky, but it also explains the blueness
that you can observe in "Tyndall effect".
And to demonstrate what Tyndall effect is,
we need a little experiment.
Take a glass of clean water, mix in some few
drops of milk, just enough to make it murky,
and try to observe it under a relatively white
light.
Notice how the mixture appears to be slightly
bluish in color?
If it is not that obvious for you, you may
use a white flashlight or any other source
of a bright white light, and shine it through
the mixture.
(I have to lower the exposure of my camera
here so that we can clearly see the mixture.)
Now it might be more obvious for you that
the mixture does indeed have a bluish color, right?
This is because the milk particles that are
suspended in the water, scatter the blue light
more strongly than the other colors in the
white light, making the whole mixture to look
slightly bluish!
This is also the reason why sometimes, the
smoke from the engine of motorcycles and some
cars looks bluish in color.
This is the same as what’s happening in
our atmosphere as the light from the sun passes
through it.
The only difference is, instead of milk particles,
it is the air molecules themselves (specifically
the nitrogen and oxygen gas molecules, which
are way too smaller compared to milk particles)
that scatter the blue light.
The difference in the particle size is also
the reason why Rayleigh scattering gives a
more intense blue color than the Tyndall effect.
Now, another interesting thing that we can
observe in our experiment is, if we try to
look back directly to our light source through
the mixture that we made, (I’m lowering
the exposure of the camera again), we would
see that the light, is now more yellowish
in color.
You can clearly see here how the white light
appears yellowish as it passes through the
mixture.
We can also try to slowly move our light source,
like this, and observe its color as it passes
through the mixture.
Notice how the light that is hitting our backdrop
changes in color?
From the original white light, it shifted
its color to a more yellowish light, as it
passes through the mixture.
This is because the blue light, from the original
white light, is being scattered in all directions
as it makes through the milk particles.
The remaining colors in the white light, however,
remains relatively undisturbed, making its
way through the mixture and hitting our backdrop,
and since it now has a lesser blue color,
it now appears more yellowish.
Again, this is the same as what’s happening
in our sky, but with air molecules instead
of milk particles.
Our sun appears yellowish to us because, as
the white light of the sun makes it through
our atmosphere, the blue light is being scattered
strongly by the air molecules, making the
direct sunlight to be less bluish and more
yellowish in color, as it reaches the ground.
It is just our atmosphere that makes the sun
to appear yellow.
The sun is originally white when seen from
the outer space.
This is also the reason why we see an orangey
or even a reddish sunlight during sunset.
When the sun is directly above the sky, it
appears slightly yellowish in color due to
our atmosphere.
But as it approaches the horizon, its light
has to pass through a thicker atmosphere before
it reaches your eyes, therefore, more blue
light is being scattered away and making the
sun to appear more orangery or reddish.
And that’s it!
The sky is blue because the air molecules
in our atmosphere scatter light from the sun,
and the shorter the wavelength of the light,
the stronger it is being scattered.
And since the shortest wavelength in the visible
light is violet, it is being scattered the
strongest by the atmosphere and therefore
making our sky blue.
...wait, what?
If the Rayleigh scattering tells us that the
shorter the wavelength of the light, the more
it is being scattered by the air molecules,
then would it be violet that should be scattered
the most?
Shouldn’t we have a violet sky rather than
a blue sky?
Well, that’s a good point!
However, we should also consider the fact
that the sun produces a very weak violet light.
To show you what I mean, take a look at this
graph.
This is the spectrum of the sun, as measured
from space.
This graph shows the intensity of every wavelength
of light that our sun produces.
The black portion here represents light from
our sun having wavelengths that are invisible
to the human eyes.
This portion of the graph here represents
light having wavelengths that are visible
to the human eyes.
The visible light!
Now, take a look at this portion of the graph
very closely.
We could see here that the color violet, or
the light having a wavelength close to 400
nanometers, has a very low intensity compared
to the other colors of the visible light.
This means that, the sun produces a relatively
weak violet light, and therefore, even if
it is being scattered stronger than the blue
light, it is being “overpowered” by the
intensity of the blue light, making the violet
light almost unnoticeable.
Another thing that worsens the visibility
of the violet light in our sky is the fact
that our eyes don’t have a receptor dedicated
in detecting violet light.
You see, our eyes only have three types of
color receptors, namely red, green and blue
receptors.
And the reason why we can see other colors
aside from just red, green and blue is through
the different combinations of these 3 types
of receptors.
For example, when a violet light (or light
having a wavelength of 400 nanometers) enters
our eyes, it triggers the red and blue receptors,
and those receptors send a signal to our brain,
through the optic nerve, which then the brain
interprets the signal as if we are seeing
a color violet.
Now, if we see another light that is now composed
of two different colors of light say red and
blue light, it will trigger again the same
red and blue receptors in our eyes, and our
brain would still interpret this as if we
are seeing a color violet, even though this
time, there is really no true violet light
present.
Your eyes will not notice the difference between
what is a true violet light (that is, a light
having a wavelength of 400 nanometers) and
what is a light that is just a combination
of red and blue light (or a combination of
light having wavelengths of 700 and 480 nanometers
respectively).
This is also the reason why individual pixel
in all most all digital screen today only
has a red, green and blue light, to display
a wide range of different colors.
Now, to demonstrate how this limitation of
our eyes affects how we see the color of the
sky, let’s try to reproduce the color of
the sky using only red, green and blue light.
When the intensity of the red, green and blue
light is at 100 percent, like this,
we can see here that we have produced white!
This means that when the red, green and blue
receptors in our eyes are equally stimulated,
our brain interprets it as if we are seeing
white!
You can also see here how the different combinations
of red, green and blue light can produce other
colors of light as well, such as yellow,
magenta,
and cyan.
The different combinations and intensities
of these red, green and blue light create
various other colors.
For example, to produce the color violet using
only red, green and blue light, we’ll have
to combine a significant amount of blue light,
and a little amount of red light, like this.
Now, we have produced a violet color!
Now, let’s go back to the color of our sky.
Since the air molecules in our atmosphere
scatter blue light very strongly because of
its short wavelength, to simulate that, we
should increase the intensity of our blue
light here.
Like this.
Obviously, this pure blue color is far from
the actual color of our sky, right?
This is because we should also consider the
fact that the other colors of light from the
sun are also being scattered by the air molecules,
it’s just not as strong as how the blue
light is being scattered.
The next strongest color that is being scattered
in the atmosphere after blue, is green,
and then the color red is being scattered the least
since red has the shortest wavelength within
the visible light.
So if we add this amount of green light, and
a little bit of red light, then we have produced
a color that is pretty much closer to the
actual color of our sky!
Now, here’s the moment of truth, imagine
what will happen if we will add a weak violet
light to this color?
Remember, adding a violet light means we should
increase the intensity of our blue and red
light.
However, we must also remember that our sun
produces a weak intensity of violet light
as what we’ve seen in the sun’s spectrum,
so we should just increase the intensity of
our blue and red light by a small amount,
like this.
We have just produced a color that is still
pretty much the same as the actual color of
our sky!
So you see, the weak violet light is actually
in there, in our sky, it’s just too weak
to be obvious, plus the fact that it blends
very well with the other colors in our sky,
that’s why we can barely notice it.
So, the next time a child or maybe a friend
asked you why the sky is blue, you can answer
it like this:
The sky is blue because of the phenomenon
called the Rayleigh scattering, wherein the
air molecules in our atmosphere scatter the
different colors of light from our sun, and
blue is being more scattered compared to other
colors, since it has a short wavelength.
Violet light has even shorter wavelength compared
to blue light but since our sun produces a
weak violet light, it becomes difficult to
be noticed (at least for human eyes) and therefore
the blue is still the color that dominates
our sky.
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