Why the sky is blue?
Abstract
Are you interested about the nature phenomenon? Are interested in physics? Are you
interested in solving problems and discover new phenomena? If you are, please pay
attention to this passage. In this passage I will explain to you why the sky is blue. I
will use some charts, principles, examples and a specific experiment to solve the
question. Then I will show you some different nature phenomena by using the
principle of the blue sky. You will know more about nature and physics after reading
this passage.
Introduction
In the early morning, you open your eyes while the bell is ringing. World
tends to be quieter and less crowded, and the first rays of daylight can
be golden and beautiful. So a new day begins. But in most cases, you are in a hurry
and you are engaged in working and studying. As a consequence of this, you ignore a
lot of nature phenomenon and exquisite landscapes. But if you set out on your quest
and slow down, you will discover that the sky over your head is so blue, especially
after a big rain. So have you ever thought of this question-----Why the sky is blue?
The phenomenon is so normal that you may have known about it since you were a
little child, You may think this is a boring question and you may think that the reason
why the sky is blue is just because the god wants the nature more colorful and
beautiful. But in this passage I will use some specific charts, experiments, and some
principles to tell you some details about it. You will have more interests and a better
understanding about this question after reading this passage.
Details
Before explaining this question we need to know a basic principle. The principle is
the scattering of light. The definition of light scattering is that when a beam of light
shines through inhomogeneous media, part of the light will deviate from theoretical
direction and spread. So you can also see the light from the side. As we all know,
when a beam of light shines through the air with dust or colloidal solution, you will
see the phenomenon of scattering. So when a beam of sunlight is shining through the
air, it can not get rid of scattering because there are a large number of air molecules
and dust particles in the air.
Normally the light scattering can be divided into two categories due to the different
reasons of light scattering. The first one is Tyndall effect. This one means that when a
beam of light shines through a colloid, we can see a bright path from the vertical
direction of the incident light. This phenomenon happens when the particles are
smaller than the wavelength of the incident light. Besides, the intensity of the light
scattering has no business with the wavelength of the light, As a result, you can also
see the light is white after scattering. For instance, we can see the phenomenon when
we look at the horizon. Besides, this one is also called the scattering of some muddy
medium.
The second one is Rayleigh scattering. The prerequisite of this phenomenon is that the
particles are much smaller than the wavelength of the incident light. It has the
following features:
① the scattering intensity is inversely proportional to the biquadrate of the
wavelength.
② The forward and backward of the scattering light is the strongest.
③ The Luminous flux of the first half of the particles equal to the rest.
④ The Polarization state of the forward scattered light is the same with the backward
scattered light.
These features are closely related with the question. And this one means that a beam
of light should shine through clean and pure medium. The Rayleigh scattering is the
basic principle of explaining why the sky is blue.
Here we come back to our question. First of all we must know that the atmosphere
around the earth has no color. It is the air particles, ice crystals and water-drops that
make the sky colorful. The reason we have discussed above, it is because of the light
scattering.
But why the sky is blue but not any other colors?
As we all know the sunlight is white. But it can be divided into seven colors. They are
red, orange, yellow, green, cyan, blue and purple. They have different wavelengths
and different frequencies. So their functions are different from each other. For
example, when we make an experiment, we can use a prism to resolve different beams
of lights. Since their wavelengths are different, some beams of lights may come
through but others cannot.
To explain this question, let me make an analogy. Let us assume light is wave. When a
wave is moving, it will spread around. If the wave meets a tiny stone or something
else that can stop it from mowing. The wave will rebound and change its direction. So
the sunlight shines through the air, it will meet a lot of obstacles such as air particles,
water-drops and some dust. When the sunlight meets these obstacles, they will
rebound like a wave and change its direction.
But you may still wander that why we still see blue even if many beams of light are
stopped by obstacles?
As we have discussed, different lights have different wavelengths, some of them have
many long wavelengths like red and orange. Their wavelengths can be over 600nm
and the red light’s wavelength can be 760nm. But some of them are very short like
blue and purple. Their wavelengths are less than 500nm. The purple light’s
wavelength can be 350nm(You can see the details from Fig1).
Let us use the same analogy, the light which its wavelengths are long is a big wave.
They can find ways around obstacles. But the light which wavelengths are short is a
small wave. They can not overcome the difficulty, so they will spread everywhere.
Just like a small wave, a beam of blue light can not come through the air fluently
because of its short wavelength, so it spreads everywhere. At last we see the sky is
blue.
So you now know that why the sky is blue. Then let us make an interesting and easy
experiment.
We need the following equipments:
① a cup of water
② some milk
③ a torch
Then we need to follow the following steps:
① We need a dark environment
② We put the cup of water in the environment and put a drop of milk into the cup
③ Turn on the torch and put it on one side of the cup
④ Then keep putting the milk and observe the phenomenon
(You can see the experiment from Fig2, and you will know exactly what the
experiment means.)
This is an experiment that verifies the blue sky, when you put a drop of milk you will
see the torch light in the water is blue. But when you continue putting the milk in the
cup, you will see the water is becoming white. And we can see this phenomenon at the
horizon.
So are you interested about the question now? I am sure you will. Using the principle
of blue sky, we can explain many nature phenomena.
First of all, I will introduce the sky when the sun is setting. In the afternoon, you feel
tired after a day’s working and studying. Then you stand up and walk around. But
when you stretch your body and look at the sky. Surprisingly, you find the sky is not
blue. It is red and orange. Why the sky is not blue? That is weird. It is not correspond
with the principle!
Actually, it is correspond with the principle. When we meet different situations, we
should solve them in different ways. Because when a beam of sunlight shines through
the air in the afternoon, they meet more particles. As a result, most blue and purple
lights spread everywhere. But the red and orange lights can not be stopped by these
obstacles. So they come into your eyes at last. That is why the sky is red and orange
but not blue.
Secondly, I will tell you the sky after sunset. When you go off the work or the class is
dismissed, you are busy going back home or getting on the last bus. But have you ever
slowed down and looked at the sky over your head? If you have, have you ever
noticed the tiny difference about the sky?
Then you may discover that the sky is dark blue. So now you have a new question.
Why the sky is dark blue now?
In fact, this is a problem which troubles many scientists, but now I can explain it to
you. This time I will introduce you a new substance first. Its name is called ozone
layer. The substance is very important to the earth. The ozone layer can absorb most
of the ultraviolet rays from the sunlight. As we are know that ultraviolet rays are
extremely radioactive and they really do harm to our bodies and the environment. But
since we have invented air-conditioners, the ozone layer is not as good as before. It is
becoming thinner and thinner because the air-conditioners can produce a lot of Freon.
The substance harms the ozone layer badly. Hence we have a thinner ozone layer in
the world.
Let us come back to the point. When the sunlight shines through the air, they meet the
ozone layer. For the ozone layer have the strong ability of absorbing the red and
orange light They let the blue and purple lights coming through it very quickly. At last
we see the dark blue sky after sunset. After all the lights disappear in the dark, the
night comes.
Thirdly, I will tell you the sky in the morning. The sky in the morning sometimes is
red and orange. Why? The principle of this is the same with the first one. You may
have guessed that. Some red and orange lights can not stopped by the particles and
water-drops. So the sky is red and orange.
Last but not least, I will tell you something about the rainbow. The principle of this is
the same with the blue sky. It is the light scattering.
We an usually see the rainbow after a big rain. But the most important condition is
sunlight because the colors come from the sunlight. You may guess that the next
condition is the water. When a beam of sunlight shines through the water, Refraction
and reflection happens. Due to this, the sunlight scatters into different colors. Because
there are usually many water-drops in the air, we can see a beautiful rainbow over the
ground.
These are the different phenomena explained by the principle of blue sky. All in all,
the principle of all these phenomena is the light scattering, especially the Rayleigh
scattering.
Conclusion
After reading this passage, do you have more interests about the physics and the
nature?
So I will make summary here.
① The principle of the light scattering is very important, it is the key to solve the
question.
② We should know that lights have different wavelengths. It is their wavelengths that
make the colorful sky.
③ At the horizon, we can see the white color because there are more particles there,
just like the milk in the experiment.
④ The sky is not always blue, it is determined by the numbers of the particles and
the ozone layer.
⑤ All the things have their own principles and laws, you should pay attention to
them and find the nature of it, then we can use them to solve many problems and
phenomena.
All in all, the blue sky is a very normal phenomenon. You even have discovered that
since you are a little child. But there are many principles to discover. In this passage
we tell you all the details about it and solve many phenomena by using the principle.
So the nature of things is very important for us. Once we find it, we can use it solve
many similar questions. And only we do this can we have a more clever mind and a
glorious future.
References
Baidu and Wiki
Fig1
The wavelength of light
Fig2
The experiment