Why is the sky a blue sea and why is it green?

The clear sky is blue, not because the atmosphere itself is blue, nor because it contains blue substances, but because atmospheric molecules and tiny particles suspended in the atmosphere scatter sunlight. Due to the inhomogeneity of the medium. The phenomenon that light deviates from the original propagation direction and scatters sideways is called the scattering of light by medium. The scattering of fine particles follows Rayleigh's law: the intensity of scattered light is inversely proportional to the fourth power of wavelength. When sunlight passes through the atmosphere, violet light, blue light and cyan light with shorter wavelength are most easily scattered, while red light, orange light and yellow light with longer wavelength are less scattered. Due to this comprehensive effect, the sky appears blue. Why is the rising sun red? In the morning, sunlight passes through the thick atmosphere. At this time, violet light and blue light are strongly scattered. When they reach the horizon, there is very little left, only yellow light, orange light and red light with longer wavelength are left. So the rising sun is red. These colored lights are scattered by atmospheric molecules, dust, water droplets and other impurities above the horizon, making the sky there show gorgeous colors. If there are clouds, the light will be reflected back, and the clouds will be colored, and sunrise and sunset will appear.

This is because the sun's rays are scattered by atmospheric molecules and particles suspended in the atmosphere after being emitted into the atmosphere. The blue of the sky can only be seen at low altitude. With the increase of height, as the air becomes thinner and thinner, the number of atmospheric molecules decreases sharply, the light scattered by molecules gradually weakens, and the brightness of the sky becomes darker and darker. At an altitude of more than 20 kilometers, the scattering effect occurs.

According to the determination of scientists, the wavelengths of blue light and purple light are relatively short, which is equivalent to "wavelet"; The wavelengths of orange light and red light are relatively long, which is equivalent to "big waves". When encountering obstacles in the air, blue light and purple light are scattered all over the sky because they can't cross those obstacles. That's how the sky turns blue. The scientist who discovered this "scattering" phenomenon is called Rayleigh. He discovered it about 130 years ago, and he is also a Nobel Prize winner. With the phenomenon of "scattering", the following astronomical phenomena can be explained. For example, the sky above you is blue, but on the horizon, where heaven and earth meet, the sky looks almost white. Why? This is because sunlight travels much farther in the air from the horizon to where you are than directly from the air-in the process, it will naturally eat more particles. These large particles scatter light many times in this way, so it appears light blue in white. I suggest you do a small experiment to verify it: take a glass of water, put it in a dark background, put a drop of milk, then illuminate one end of the cup with a flashlight and * approach it. The light of a flashlight will appear light blue in water. If you put more milk into water, the water will be whiter, because the light is repeatedly scattered by many milk particles, and the result is white. It is as white as the horizon. When the sun goes down in the evening, the sky turns red instead of blue, and the setting sun turns dark red. Because the light in the evening meets countless particles on the way to your place, the purple and blue parts of the sun are scattered in all directions, leaving only a little orange-red light visible to the naked eye-because they have a long wavelength. "

If the sun falls from the sky, it will constantly encounter some obstacles-even if it doesn't rain. Because the air that light must penetrate is not empty, it is made up of many tiny particles. Most of them, 99% are either nitrogen or oxygen, and the rest are other gas particles and tiny floating particles, which come from automobile exhaust, factory smoke, forest fires or volcanic ash. Although the particles of oxygen and nitrogen are one million times smaller than a drop of rain, they can still block sunlight. Light bounced off by these small stumbling blocks and changed its direction: light was scattered out, which is what we chemists and physicists say. Short-wavelength blue light and purple light scatter much more than long-wavelength orange light and red light. So in scattered light, violet light is almost 10 times that of red light, while blue light is almost 6 times that of red light. Green, yellow and orange light cannot compete with the dominant blue and purple light, so we think these scattered lights are blue sky. It was discovered by Lord Rayleigh, a British physicist and Nobel Prize winner, 130 years ago: when light deviates from its original straight direction in air, the wavelength of light is different, and the deviation distance is also different. Later, in honor of him, people called this scattering process Rayleigh scattering. If you look at the sky, you mainly see the blue light scattered in the sunlight, not the undiffused sunlight.

2. Why is the sea blue?

The color of seawater seen by human eyes is the color of seawater reflecting light from the sun. When white light shines into seawater, seawater appears blue because of its selective absorption and scattering. When light passes through the medium, part of the energy of light is absorbed by the medium and converted into the internal energy of the medium, so that the intensity of light decreases with the thickness of the medium, which is called light absorption. If a medium absorbs light in a certain wavelength range to a small extent and does not change much with the wavelength, this absorption is called generalized absorption; If a medium absorbs some wavelengths of light very strongly and varies greatly with the wavelength, this absorption is called selective absorption. When sunlight shines on seawater, seawater selectively absorbs red and yellow light, but strongly scatters and reflects blue and purple light, so seawater looks blue. The color of most objects is the result of selective absorption of visible light on the surface or in the body. The sky we see on a sunny day is blue. Everyone may have noticed that sometimes after a heavy rain, the sky will become particularly blue, and the clearer the weather, the bluer the sky. There is a simple reason. The scattering of sunlight by the atmosphere makes the sky look blue.

The surface of the earth is surrounded by the atmosphere. When sunlight enters the atmosphere, air molecules and particles (dust, water droplets, ice crystals, etc. ) will scatter sunlight around. Sunlight consists of seven kinds of light: red, orange, yellow, green, blue, indigo and violet, of which red light has the longest wavelength and violet light has the shortest wavelength. Red light with longer wavelength is the most transparent and can directly penetrate particles 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. In the short-wave band, blue light has the largest energy and the most scattered light waves, so the sky we see is blue.

In fact, the sky has always been blue. You can see the blue sky almost every day on the plateau. It is windy and dusty in spring, cloudy in summer and foggy in winter, which makes it impossible for us to see the blue sky often. Only in autumn, the air is clean, which gives us many opportunities to see the blue sky.

The color of seawater is determined by the color of reflected light from the sea surface and backscattered light from the seawater. Because blue light and green light have the strongest penetrating power in water, there is the greatest chance of backscattering. Therefore, the sea looks blue or green.

Sunlight is projected onto the sea surface, part of it is reflected, and the rest enters the water. When the sun shines vertically on the sea, the reflected light is very small, only about 2% on the calm sea. As the sun approaches the horizon, the reflected sunlight gradually increases. In fact, the amount of sunlight entering the sea varies with many factors, such as the projection angle of the sun, weather conditions, sea surface conditions and the clarity of the sea water. Sunlight is composed of light with different wavelengths, and seawater is selective in absorbing and scattering light with different wavelengths. Sea water absorbs the most red light and transmits the most blue light. Most of the red light can only penetrate the water layer of 2 to 3 meters. Blue light penetrates the deepest, exceeding 500 meters.

In addition, suspended particles in seawater absorb more blue light and green light with shorter wavelength, while the scattering of other light has nothing to do with the wavelength of light. The color of seawater is mainly determined by the scattering of light by water molecules and suspended particles, so the color of seawater with different turbidity is different. There are many suspended particles in coastal waters, and the particles are also large. Therefore, from the far sea to the offshore, the color of seawater gradually becomes lighter from dark blue. Near the estuary with high sediment concentration, there are a lot of yellowish substances produced by land plants decomposition in seawater, so the seawater looks pale green. Why the sky is blue: This is the refraction of the sun light by countless small water droplets in the air, and then blue is reflected on the human retina. In fact, the sky outside the earth is black (because there is no refraction of light by air and moisture).

The same can be proved: Why is the sea green? It should be noted that the sea is usually blue and the lake is more likely to be green. )

In addition, add a little extra-curricular knowledge: take a prism to reflect sunlight, you can see the seven colors contained in the sun, and you can know the principle of rainbow.