There are so many colors in the sky, why do we usually see only blue? Here are some reasons why the sky is blue. I hope it can help you. First of all, we need to know a truth: the reason why things around us show color is because the sun shines on them. Although the sun looks white, all the colors: red, orange, yellow, green, cyan, blue and purple exist in the sun.
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The reason why the sky is blue
First of all, we need to know a truth: the reason why things around us show color is because the sun shines on them. Although the sun looks white, all the colors: red, orange, yellow, green, cyan, blue and purple exist in the sun.
Sunlight from the sky down, will continue to encounter certain obstacles. There are nitrogen, oxygen, gas particles and tiny floating particles and so on. When the light meets these obstacles, it naturally changes its direction.
The wavelengths of blue and purple light are shorter, while the wavelengths of orange and red light are longer. When encountering obstacles in the air, blue light and purple light are 'scattered' everywhere because they can't pass through those obstacles, covering the whole sky. That's how they are 'scattered' into blue.
Rayleigh, the scientist who discovered this phenomenon, discovered it 130 years ago. He is also a Nobel Prize winner.
The scattering phenomenon can be used to explain the following phenomena at the same time.
For example, the sky above you is blue, but where the horizon meets the sky, it looks almost white. This is because the sun's distance from the horizon to your place is much longer than its distance from the sky, and it naturally rubs more particles along the way. So many times these particles scatter light, so it looks white and blue.
It is suggested that you do a small experiment to verify: take a glass of water, put it in a dark background, put a drop of milk in it, and then take a flashlight to irradiate one end of the glass, and close to it, the light of the flashlight will show light blue in the water. The more milk you put into the water, the whiter the water will be, because the light is repeatedly scattered by these numerous milk particles, and the result is white. It's the same as white over the horizon.
In the evening when the sun sets, the sky does not appear blue but red, and the setting sun also turns dark red, which is the same reason. Due to the numerous particles that the evening light encounters on the way to your place, the purple and blue parts of the sunlight scatter in all directions, leaving only a little orange red light that can be seen by your naked eye. Because of their long wavelength and 'big wave', they cross the obstacles on the road.
In fact, the same is true from outside the earth. The sea water covering two-thirds of the earth also emits blue light. Although there is brown land or green forest on the land, the sky is always blue. From the perspective of the universe, the whole earth is wrapped in a soft blue veil.
Blue is the color of life. Our earth is the blue planet. To protect our earth is to protect our lives.
Principle: light scattering refers to the phenomenon that a part of light deviates from the original direction when light passes through uneven medium, and the light deviated from the original direction is called scattered light. The clear sky is blue, which is the result of atmospheric molecules and tiny particles suspended in the atmosphere scattering the sunlight. The violet, blue and cyan light with shorter wavelengths are most easily scattered, while the red, orange and yellow light with longer wavelengths are less scattered. This comprehensive effect makes the sky appear blue.
Xueba analyzes why the sky is blue
So why is the sky blue? The answer is Rayleigh scattering.
Rayleigh scattering refers to the scattering effect of particles smaller than the wavelength of light on the incident light. Note that we are talking about particles smaller than the wavelength of light. The wavelength of visible light we can see is 390 ~ 700 nm. Water droplets, ice crystals and dust obviously do not belong to this category, while molecules in the air belong to this category. In other words, even if the air is pure without any impurities, we can see the blue sky. If the blue sky is caused by the scattering of large particles, the more serious the haze is, the bluer the sky is. This obviously doesn't make sense.
So why can Rayleigh scattering make the sky look blue? According to the formula of Rayleigh scattering, the light with shorter wavelength (blue and purple) is easier to be scattered, while the light with longer wavelength (red and orange) is not easy to be scattered. Therefore, when the sunlight passes through the atmosphere, more blue and purple light diffuses into the whole sky due to Rayleigh scattering, so the sky looks blue.
Rayleigh scattering can also explain why the sun we see is yellow. Because a lot of blue and purple light is scattered in the atmosphere, there will be less blue and purple light directly reaching the ground, so when we look up at the sun in the sky, it will be warm yellow. Sunrise and sunset look red, which is also affected by Rayleigh scattering, because near the horizon, the sunlight needs to pass through a thicker atmosphere to reach us. Because the atmosphere is very thick, even the yellow and green light with relatively long wavelength is scattered, so we see more red and orange light. In fact, the sun seen in space is white.
Rayleigh scattering makes the sky blue
We know that the light coming directly from the sun looks white at noon. It consists of a continuous mixture of visible wavelengths with peaks in the green yellow region of the spectrum. So why do we see the blue sky? What causes the orange red we see when the sun sets? These phenomena are related to a process called scattering.
Before we think about the color of the sky, we have to ask how we see the sky. Where does the light in the sky come from? The light in the sky comes from the sun, but it is scattered by direct sunlight. We can describe scattering as a process in which light is absorbed by particles in the atmosphere and emitted again rapidly at the same wavelength. But the direction of the scattered light is different from that of the incident light.
If there was no atmosphere, there would be no scattering. Then the sky will be black with no light. However, the earth sometimes has an atmosphere, and the troposphere extends about 10km above the earth's surface. The earth's atmosphere is mainly composed of nitrogen, oxygen and a small amount of other gases. In addition, you can swim freely in soot particles, volcanic ash or other particles. These particles are very small, but still much larger than individual gas molecules.
The scattering of gas molecules is the main reason for the blue sky. When the size of scattering particles is smaller than the wavelength of light, the scattering process is called Rayleigh scattering, which is named after Lord Rayleigh (William Thomson). Rayleigh scattering changes rapidly with wavelength, and the scattering efficiency of shorter wavelength is much higher than that of longer wavelength.
We can think of gas molecules as tiny antennas. Because gas molecules are composed of charged particles, when the electromagnetic wave hits the molecules, these charges will vibrate at the frequency of the wave. Just as the oscillating current generates radio waves, the scattered light waves are generated by the oscillating current in the gas molecules. When the wavelength of the wave is approximately the same as the size of the antenna, the efficiency of this process is the highest. Because the size of gas molecules is only a few nanometers, and the wavelength of visible light is hundreds of nanometers, the efficiency of the scattering process is not high. However, the shortest wavelength in the blue region of the visible spectrum is more efficient.
That's why the sky is blue. Blue light is more efficient at scattering from the sun's direct beams than red or other intermediate wavelengths. Light needs to be scattered many times to reach our eyes.
When we look at the sky, we don't look directly at the sun. What we see is light that has been scattered many times, concentrating shorter blue and purple wavelengths in the light that reaches our eyes. Because the spectrum of sunlight contains more blue light than purple light, and the eye's response to the blue wavelength is stronger than that to the purple wavelength, the color we recognize is blue.
Why does the sun look orange or red near sunset or sunrise? At sunset, the distance that the sun's direct light travels through the earth's atmosphere is much longer than at noon. Because blue light and intermediate wavelength light are scattered out of the beam with higher efficiency than red light, the rest of the direct beam is mainly red wavelength. The closer the sun gets to the horizon, the redder it looks.
Larger particles, such as water droplets in clouds, can also scatter. The size of water droplets is usually larger than the wavelength of visible light. In this case, the scattering intensity has little to do with the wavelength. As a result, the light scattered from the cloud is white or gray. All wavelengths are equally scattered, resulting in the same color as the incoming sunlight, but with weaker intensity.