Why is the color of the sky white?

The sky we see is often blue, especially after a heavy rain. The sky is as blue as autumn water, which makes people feel relaxed and eager to fly. Why is the sky blue?

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.

Why is the sky blue, not green or red?

First of all, you have to understand a truth: the things around us are colored only because the sun shines on them. Although sunlight looks white, all colors: red, orange, yellow, green, cyan, blue and purple exist in sunlight.

There are so many colors in the sky, why do I usually only see blue? You might ask.

If you think of light as a wave, you will solve the mystery. Light actually moves like a wave. Let's imagine a drop of rain falling in a puddle. When this drop of rain falls on the water, it will produce small waves, which will fall into a larger circle and spread in all directions. If these waves hit pebbles or other obstacles, they will bounce back and change the direction of the waves.

When the sun falls from the sky, it will continue to encounter some obstacles. Because the air that light must penetrate is not empty, it is made up of many tiny particles. Ninety-nine percent of them 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 oxygen and nitrogen particles are only one millionth of a drop of rain, they can still block the path of sunlight. The light bounced back from these small stumbling blocks and naturally changed direction.

But so many colors of light have changed direction, why only see blue? You probably still don't understand.

We must go back to the puddle we just talked about.

In a puddle, if a small wave meets pebbles, the water surface will be chaotic; But if it is a "huge wave", just like the kind of "huge wave" that you lift by hand at the edge of the puddle, it may simply overflow from the stone and reach the edge opposite the puddle unimpeded. Then, just like there are big waves and small waves, all kinds of light waves have different "waves", that is, wavelengths: but unlike water waves, their size is invisible to the naked eye, because they are incredibly small, only 1% of a hair! You have to use very sensitive measuring instruments to measure accurately.

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" everywhere because they can't cross those obstacles, covering the whole sky-the sky, so they are "scattered" into 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 meets the earth, the sky looks almost white. Why? This is because sunlight travels far in the air from the horizon to where you are than it falls directly from the air-and it naturally eats more particles along the way. 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, and then illuminate one end of the cup with a flashlight and get close to 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. As the sunset met countless particles on the way to your place, the purple and blue parts of sunlight scattered in all directions, leaving only a little orange-red light visible to the naked eye-because of their wavelength and "big waves", they have crossed the obstacles on the road.

However, if you are careful, you will find that the sky will appear dark blue for a period of time after sunset. This used to be a strange thing that scientists cared about, but several physicists solved the mystery 50 years ago: the blue color that caused the dusk sky was a special substance. This special substance gathers into a thick layer at a height of 20 to 30 kilometers from the earth's surface, which is called the ozone layer. This gas acts as a color filter for the falling sunlight: it intercepts the yellow and orange parts of the sunlight, but lets the blue part pass almost unimpeded. When the last light disappears, all the colors disappear in the night.

Ozone not only creates the blue sky at dusk, but also swallows a special kind of light that you can't see: ultraviolet light, or ultraviolet light. You must have heard how dangerous ultraviolet rays are to all living things, including you. If it shines on your bare skin for too long, you will get sunburned. The ozone layer everywhere is thick enough to intercept as much ultraviolet rays as possible: this is extremely important for all life on our planet.

Unfortunately, today, this life-threatening protective layer has become thinner in many places, and even a big hole has been formed over the South Pole. The killer of ozone destruction is "Freon"-a substance that people use to spray hair mousse or for refrigeration in refrigerators and air conditioners. This is a substance that is particularly harmful to the ozone layer, so many countries no longer use this "ozone killer".

Today we know why the sky in our eyes is blue. In fact, it is the same from the outside of the earth: the sea water covering two-thirds of our earth also emits blue light. Although there are brown land or green forests on the land, the sky is always blue-from the perspective of the universe, the whole earth is wrapped in a soft blue veil. Astronomers who have seen the earth from outside the atmosphere have reported this situation.

So it is absolutely right that the earth is called "blue planet". Its unique blue is the color of life.

And:

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.

Why is the sky blue?

The blue sky we see is the result of selective scattering of visible light in incident solar radiation 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 visible light radiated by the sun 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.

If light with short wavelength scatters more strongly, you must ask why the sky is not purple. One of the reasons is that when the visible light radiated by the sun passes through the atmosphere, air molecules have a strong absorption rate of violet light, so we observe less violet light in the visible light radiated by the sun, but it is not absolutely absent. 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 are the opposite of blue and purple.