The sky was originally blue.

1 Why is the sky blue? This is because sunlight is composed of seven colors: red, orange, yellow, green, cyan, blue and purple. The wavelengths of these seven colors are different. Compared with other photons with longer wavelengths, dust and other particles in the atmosphere scatter blue light more, so the sky looks blue. There are two main types of light scattering in the atmosphere: Dindar scattering and Rayleigh scattering. Among them, dust, water mist and other particles that can form colloid in the air scatter light by Tindal scattering. The characteristic of Dindar scattering is that the intensity of scattered light has nothing to do with the wavelength of light waves, so white light is still white after scattering, and white seen near the horizon is Dindar scattering. There is also Rayleigh scattering, which is produced by tiny particles (molecules, atoms, etc.). ), its scattered light intensity is inversely proportional to the fourth power of light wave wavelength. It is known that the wavelength range of visible light is 400nm (blue violet) to 700nm (red light), and the wavelength of red light is 1.75 times that of blue violet, so the scattering intensity of blue violet light is nearly ten times that of red light, and human eyes are insensitive to purple light. The atmosphere itself is colorless. The blue of the sky is a picture created by atmospheric molecules, ice crystals, water droplets and sunlight. When sunlight enters the atmosphere, long-wavelength colored light, such as red light, has great transmission power and can penetrate the atmosphere and shoot to the ground; However, violet, blue and cyan light with shorter wavelength will easily scatter when it touches atmospheric molecules, ice crystals and water droplets. Scattered purple, blue and cyan light fills the sky, making it appear blue. According to Rayleigh theory, when the wavelength of light wave decreases, the scattering degree increases sharply. Therefore, violet light with the shortest wavelength should be the strongest scattering, while indigo, blue and green light scattering is much smaller. So why do we see the blue sky instead of the purple and indigo sky? It turns out that when scattered light passes through the air, it will lose a lot of energy because of absorption. Violet light and indigo light with short wavelength are strongly scattered when passing through the air, but they are also strongly absorbed by the air. When the sun reaches the ground, the remaining purple and indigo light is not scattered much. The color of the sky we witnessed is a mixture of colors close to blue in the spectrum, and they present the color of the blue sky. The sky has no color, but it has color because of the scattering of sun light. That's why the night sky looks black. At night, the sun shines less than half of the earth's sky. Without strong sunlight, the sky has no color, only black. The magnificent scene of the stone is difficult for any artist to describe. But few people know that most of the colors we witness are caused by pollution. The sunset in the city is different from that in the countryside with fresh air. In a very clean and unpolluted atmosphere, the sunset has distinct color characteristics. The sun is bright yellow, while the adjacent sky is orange and yellow. When the sunset slowly disappeared below the horizon, the color of the sky gradually changed from orange to blue. Even after the sun disappears, clouds near the horizon will continue to reflect sunlight. Because the blue of the sky and the red sunlight reflected by the clouds blend together, the thin clouds in the higher sky appear reddish purple. After a few minutes, the sky was full of light blue, and the color gradually deepened and extended to the sky. But in a highly industrialized area, when pollutants are suspended in the air in the form of particles, the color of the sky is completely different. The round sun is orange-red, while the sky is dark red. The difference of red light and shade reflects the thickness of pollutants. Sometimes after sunset, two broad colors appear in the sky on both sides, dark red near the horizon and dark blue above. When the pollution is particularly serious, the sun looks like a dark red disk. Even before it reaches the horizon, its color will gradually fade. Why is the sun yellow in clean air and the sky blue at the same time? 1At the end of the 9th century, the British physicist Rayleigh explained this for the first time in 187 1. People on the surface of the earth observe the sky through the scattered sunlight in the air. In a clean and pollution-free atmosphere, most of the scattering is caused by molecules in the air (mainly oxygen and nitrogen molecules), and the size of these molecules is much smaller than the wavelength of visible light. Rayleigh theory points out that the intensity of scattered light is inversely proportional to the fourth power of wavelength (I ∝ 1/λ 4). In this case, scattering mainly affects light with shorter wavelength. Because blue is behind the spectrum, the sky itself looks blue. Sunlight directly penetrates the air and loses a lot of blue in the scattering process, so the sun itself appears brilliant yellow. In addition to scattering, sunlight is also absorbed by ozone molecules and water vapor in the air. Due to the interaction of scattering and absorption in the air layer, the sunlight that finally reaches the ground consumes a lot of energy. It is precisely because in the morning and evening, sunlight travels far in the air and loses too much energy that we can enjoy the magnificent sunrise and beautiful sunset. During the day, the distance of sunlight passing through the atmosphere is shorter, and its energy loss is less. At this time, looking directly at the sun with the naked eye will make people dizzy and very dangerous. Just before the sun goes down, you will see a dazzling red halo around the sun disk. This halo is the result of sunlight being refracted by dust particles much larger than air molecules, which are usually suspended in the air near the earth. This ring seems to extend about three times from the center of the sun's disk. Because the extension angle of halo depends on the wavelength of light wave and the size of particles, it is estimated that the diameter of refractive particles is about the size of dust particles. If a heavy rain purifies the air before sunset, this halo is usually invisible at sunset. Rayleigh failed to explain the problem of air pollution clearly. Although his theory points out that the scattering intensity of light will increase sharply with the increase of scattered particles, it is only applicable to particles much smaller than the wavelength of light waves, but not to particles with a diameter greater than 0.025 mm (such as air molecules). In today's industrial society, pollutants are usually suspended particles, which are composed of particles with a diameter of 0.01~10 mm. Rayleigh's theory cannot explain this situation. Later, Gostaf Mi proved that the scattering of large particles depends on the ratio of particle linearity to wavelength, and put forward a more general theory in 1908, covering a larger particle size range. This theory points out that if there are enough particles in the air, they will determine the scattering situation. Michaelis scattering theory can explain what we see in the city sky. The larger the particle, the more scattering, and the scattering effect depends on the wavelength. Scattering is strong not only in the blue region of the spectrum, but also in the green to yellow region. So the intensity of sunlight passing through many polluted air layers is much weaker, and the sun looks redder. It has lost its blue, yellow and green components. Besides scattering, light energy is also absorbed by ozone and water vapor. Therefore, the round sun appears dim and orange-red. What about the color of the sky itself in the polluted air? The pollutants suspended in the air will gather in the integrated layer for a long time, and the larger particles will form a dense layer near the ground. When sunlight penetrates these layers, it will gradually fade and appear orange. Scattered light will lose a lot of light waves with shorter wavelength, and the result is mainly red light penetration. The sky is dark red; Because the scattered red light passes through the lower and denser air in the air layer, the red color is getting thicker and thicker near the surface of the earth. The type of sunset you see mainly depends on where you are. On the ground, the brightness and color of the sunset depend on the season and the local atmospheric conditions every day. The sunrise and sunset seen by people on high places are completely different. Sometimes after sunset, the observer standing on the platform can see the sunlight scattered by a small part of the air near the two horizons. At sunrise, scattered light can be seen before the sun rises, while at sunset, the color of the sky depends on the atmospheric conditions. The bright colors in the sky before sunrise, such as orange, yellow, purple and deep blue, indicate that the atmosphere in the east is relatively unpolluted. Once the sun rises, most of the sky turns blue, and only the part near the ground appears narrow orange. The evening sky can reveal the air pollution. Natural "pollution" will also affect the color of the sky, especially when a large amount of dust, hot gas and water vapor from volcanoes enter the atmosphere. Final gathering of dust particles and other particles is in the layer between15km and 20km above the ground. The effect of this kind of air layer scattering sunlight is particularly obvious, colorful, and the sun is blue or green, especially at dusk, and this scene can be seen several years after the volcanic eruption. These charming scenery can't make up for the harm of pollution, whether it is natural or man-made. But at least pollutant particles show their existence through subtle changes in the colors of the colorful sky. Once the city appears dark red at sunset, it is a warning to us. We should prohibit pollutants from being discharged into the atmosphere. Only in this way can we ensure that our future generations can continue to enjoy the clear sky. Is the sun really moving? The sun is not static in the universe, on the contrary, it is moving in different ways. First of all, because all planets have mass, the sun rotates around the center of gravity of the solar system; Secondly, it rotates around the center of gravity of the Milky Way with the whole solar system at a speed of 250 km/s; Thirdly, it moves with the whole galaxy at a speed of 300 km/s in the local galaxy group; And it's still spinning. Q: Why is the rainbow curved when it rains, instead of a large piece of color? A: When sunlight enters raindrops and reflects from the inner surface of raindrops, rainbows are formed. As long as light from one medium, such as air, enters another medium, such as water, it will change its path, which is called refraction. The curvature of an optical path depends on the substance and wavelength it passes through, that is, the color of light. So when sunlight enters raindrops, the bending degree of different rays it contains is different: red is the smallest and purple is the largest. So all kinds of colors emerge from raindrops, and the radian of each color is about 40 ~ 42 degrees. This happens to every raindrop illuminated by the sun, but we can only see rainbows in some raindrops. Their radian is 40 ~ 42 degrees, which just transmits their light to our eyes. These eligible raindrops form a circle, some of which are often cut off by the horizon. What remains on the horizon is what we call a beautiful rainbow. Why is the sky blue? Blue sky and white clouds are floating. I believe everyone has some feelings about this beautiful scenery. Then why is the sky blue? Why are clouds white? Not everyone can tell the reason for this wonderful physical phenomenon. In fact, the beautiful scene we are watching is the result of the interaction between atmospheric molecules, water droplets and other particles in the sky and sunlight. 1. Air and sunshine In order to explain this physical phenomenon, we should first understand air and sunshine. Air is a layer of "body armor" wrapped around the earth to protect the living things on the earth from ultraviolet radiation. Air is not empty, but is made up of many particles. 99% of them are nitrogen and oxygen, and the rest are other gases (such as carbon dioxide, inert gas, etc.). ), small water droplets and floating particles such as factory dust, sand blown in the wind and rock ash produced by volcanic eruption. However, the composition of air is not fixed, it depends on location, weather and other uncertain factors (such as forests, oceans, volcanic eruptions and pollution). Light is a way for energy to propagate through electromagnetic waves, and the propagation speed in vacuum is 300,000 kilometers per second. Unlike other waves (such as sound waves), light has wave-particle duality. This is because light is composed of massless particles-photons, so light not only has the characteristics of waves, but also has the characteristics of particles. The energy transmitted by light is directly proportional to the frequency of light, and the frequency of light just determines its color. However, our eyes can only see light in a specific frequency range, which is called visible light. We can see neither too high frequency (ultraviolet) nor too low frequency (infrared). For sunlight, Newton first discovered with a prism that it contains seven colors: red, orange, yellow, green, blue, indigo and purple. You can observe "colorful sunshine" through a small experiment (as shown in figure 1). Take a glass jar filled with water and put it where the sun enters the house. Then put a small mirror in the water and use a piece of white paper to receive the light reflected by the small mirror in the basin. According to the refraction principle of light, you can see beautiful artificial rainbows from white paper. Among seven different kinds of light, red light has the longest wavelength (lowest frequency) and violet light has the shortest wavelength (highest frequency). What we see with the naked eye is the result of their mixing. 2. Why is the sky blue? Light travels in a straight line unless there is external interference. When light propagates in the air, it will inevitably encounter gas molecules and other particles in the air. These particles have physical effects on light absorption, reflection and scattering, and it is these physical effects that make the clear sky appear blue. The correct explanation of why the sky is blue begins with 1859. Scientist Tedor first discovered that blue light is much stronger than red light, which is the "Tedor effect". A few years later, scientist Rayleigh studied this phenomenon in more detail. He found that the scattering intensity is inversely proportional to the fourth power of wavelength. Later, more scientists called this phenomenon "Rayleigh scattering". Rayleigh scattering can be easily verified by the following small experiment (as shown in Figure 2): use a water cup filled with water, then drop a few drops of milk into the water cup, and use a flashlight as the light source to illuminate from one side of the water cup. From the other side of the glass, you can see red light, but from the direction perpendicular to the light, it is blue (the effect is more obvious in the dark). At that time, both Tedor and Rayleigh believed that the blue sky was caused by small dust particles and water droplets in the air, similar to the suspended particles of milk in water. Even today, many people still think so. Actually, it's not. If the sky is made entirely of small dust particles and water droplets, the color of the sky will change with humidity. In fact, the color of the sky will not change with humidity unless it rains or clouds are gathering. Later, scientists speculated that nitrogen and oxygen molecules in the air were enough to explain the "Tedor effect" in the sky. This guess was finally confirmed by Einstein, who made a detailed calculation of this scattering effect, and the calculation results were consistent with the experiments. The blue sky we see is the result of selective scattering of incident sunlight by air molecules and other particles. Scattering intensity is related to particle size. When the particle diameter is smaller than the wavelength of visible light, the scattering intensity is inversely proportional to the fourth power of the wavelength, and light with different wavelengths is scattered in different proportions, which is also called selective scattering. When sunlight enters the atmosphere, air molecules and particles (dust, water droplets, ice crystals, etc. ) will scatter sunlight around. Of the seven kinds of sunlight, red, orange, yellow, green, blue, indigo and purple, red light has the longest wavelength and violet light has the shortest wavelength. The red light with longer wavelength has the highest transmittance, and most of it can directly penetrate the particulate matter in the atmosphere and shoot to the ground. However, blue, indigo, purple and other short-wavelength colored light are easily scattered by particles in the atmosphere. Take blue light (wavelength 0.425μm) and red light (wavelength 0.650μm) in the incident sunlight as examples. When light passes through the atmosphere, the blue light scattered by air particles is about 5.5 times that of red light. So on a clear day, the sky is blue. However, when there is fog or thin clouds in the sky, because the diameter of water droplets is much larger than the wavelength of visible light, the effect of selective scattering no longer exists, and light with different wavelengths will be evenly scattered, so the sky appears white. If light with short wavelength scatters more strongly, you must ask why the sky is not purple. One of the reasons is that when sunlight passes through the atmosphere, air molecules have a strong absorption rate of purple light, so we observe less purple light in sunlight, but it is not absolute. We can easily observe the purple light in the rainbow after the rain. Another reason is related to our eyes themselves. In our eyes, there are three types of receptors, called red, green and blue cones, which are only sensitive to the corresponding colors. When they are stimulated by external light, the visual system will reconstruct the color of these lights according to the intensity of stimulation of different receptors, that is, the color of the objects we see. In fact, red cones and green cones also reflect blue and purple stimuli. The red cone and green cone are stimulated by sunlight at the same time. At this time, the blue cone is stimulated by blue light, and finally the result of their combination is blue, not purple. Have you ever seen the blue sun? You may say why the sun we see is not blue. This is because when we look directly at the sun, the sunlight received by our eyes is easily scattered by wheat, not Rayleigh scattered light. Maiyi scattering is a kind of scattering when light meets particles larger than its wavelength, which has little dependence on the wavelength of light and does not change the original composition of light. Moreover, the scattered light is forward, and most of the light still propagates in the original direction. It has the same effect on all light. When we look directly at the sun, we see a yellowish disk. Pale yellow is because in this process, some light is Rayleigh scattering, and all blue light is scattered, leaving red, orange, yellow and green light. However, compared with Mai scattering, this scattering process is weak, so the sun looks slightly pale yellow. But in sandstorm weather, because there are many particles in the air and Rayleigh scattering is dominant, we may see the blue sun. 4. Why is the sunset red? When the sun is about to set, the distance for sunlight to reach the observer through the atmosphere is much longer than at noon, and more light is scattered and reflected, so the light is not as bright as at noon. Because the light with shorter wavelength-blue and purple light-has almost been scattered, leaving only orange and red light, the sky looks from orange to red when the sun sets slowly. Similarly, when the sun rises, it is orange or red. 5. Why are clouds white? Clouds in the sky are made up of small water droplets and dust in the air. Their diameter is much longer than the wavelength of sunlight of any color, so Rayleigh scattering rarely occurs. Part of the sunlight is reflected into the air; Part of it is scattered by Michael, and then the scattered light hits the earth, but Michael scattering does not change any color of sunlight; Some directly penetrate the gaps between water droplets. The above three conditions have no effect on the composition of sunlight, so it seems that the clouds in the sky are white. But the clouds are getting thicker and thicker, and there are more and more water droplets, which are almost connected. Sunlight and scattered light cannot or rarely penetrate clouds, so white clouds become dark clouds. It is in the process of sunlight incident on the earth's surface through the atmosphere that air molecules or other particles in the atmosphere will absorb, reflect and transmit sunlight, thus forming the brilliant afterglow of blue sky, white clouds, sunset and morning glow. If there is no atmosphere and other particles, even during the day, the sun looks like a solitary bright ball, and the sky will be dark, so the air not only provides us with living conditions, but also makes our sky colorful.

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