Why is the sky blue? Because the sea is blue, this answer is romantic, but unreasonable. In fact, the blue of the sky and the blue of the ocean are not directly related. Both blues are derived from another thing, that is Sun.
But the sun clearly has seven colors, why is the sky uniquely blue? In fact, it is not accurate to say that the sun has seven colors. The colors contained in the sun are far more extensive than the seven colors we see, but the light of other colors exceeds our visible range. The sun is regarded as an "absolute black body" in physics. What is a black body? To put it simply, it is an object with strong radiation ability but weak reflection ability. The sun is such an object. It constantly radiates strong electromagnetic waves outwards at all times, but its reflection ability is very high. weak. The electromagnetic waves radiated by the sun cover a wide range of wavelengths. Among them, electromagnetic waves with longer wavelengths and electromagnetic waves with shorter wavelengths contain relatively low energy, while the parts with relatively high radiant energy appear red, orange, and orange. The seven colors of yellow, green, blue, indigo, and purple are what we usually call visible light.
Why does light with higher radiant energy happen to be visible to the human eye? This is not a coincidence, but the result of biological adaptation to sunlight in the long-term evolution process.
Since there are seven colors of sunlight visible to the human eye, why did the sky only choose blue? The reason is scattering. We know that the sun can illuminate the earth because the earth has an atmosphere. If there is no atmosphere, no matter how strong the sun is, it will only be a bright spot in the distance. Observing the sun on the moon is like this. So why can the atmosphere scatter sunlight? This is because there are all kinds of small particles in the atmosphere. These particles are too small, too small to be less than one-tenth of the wavelength of the incident light, so an optical phenomenon appears. It is called " Rayleigh scattering".
According to Rayleigh scattering,The scattering intensity of light is inversely proportional to the fourth power of the incident light wavelength, which means that the longer the wavelength, the weaker the scattering intensity.
Visible light is arranged according to the wavelength from long to short, followed by red, orange, yellow, green, blue, indigo, and purple. Among these seven colors, the light with strong scattering ability and high radiant energy is Blue light, so naturally, under the effect of Rayleigh scattering, the sky appears blue. So is the sea blue because the sky is blue? Not really.
The sea is composed of water, and water is colorless and transparent, so the color of the sea depends on the absorption and reflection of light by the water. Water has a very strong ability to absorb long-wavelength light, so the four colors of red, orange, yellow, and green are basically absorbed by water molecules, while the blue part of the light is reflected back. So the sea we see is blue, and the darker the place, the darker the color, and it may even be close to purple.
So why is the color of the sea close to the coastline close to green, or even yellow?
In addition to the optical properties, the color of the sea is also affected by suspended solids in the ocean. Because there are a large number of suspended plants in the ocean, these plants have a strong ability to absorb red and blue light, but they reflect Green light, so places close to the coastline will appear green or even yellow.
said why the sky is blue and why the sea is blue, so I have to ask another question, that is, why is the sun red? These three questions can be said to be the classic three consecutive questions children ask their parents. In fact, it is not very accurate to say that the sun is red, because only the sun in the morning and evening shows obvious red, and the sun at noon is actually bright white, so to answer this question, you can take a look at the noon. What is the difference between the sun in the sky and the sun in the morning and evening.
The biggest difference between the sun at noon and the sun in the morning and evening is the angle.
The sun at noon is high above the head, and the atmosphere penetrated by direct sunlight is relatively thin. Therefore, the seven colors of light will reach the ground and enter our eyes. The seven colors are mixed together to show It is bright white, so we think that the sun at noon is bright white.
The sun in the morning and evening is no longer above our heads, but diagonally in front of us. The angle is very large. From such a large angle, the sunlight will penetrate the thick atmosphere, so a lot of Sunlight will be consumed by the scattering of the atmosphere, and as we said before, the shorter the wavelength of light, the stronger its scattering ability, so short-wavelength green light, blue light, indigo light, and purple light are scattered. Because of its weak scattering ability, the red light with the longest wavelength penetrates the atmosphere into our eyes, and the sun we see becomes red.
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