Why is the sky blue? Because the sea is blue, this answer is very romantic, but it makes no sense, in fact, the blue of the sky and the blue of the ocean are not directly related, both of which are derived from another thing, that is, the sun.
But the sun obviously has seven colors of brilliance, why does the sky appear blue alone? In fact, it is not accurate to say that the sun has seven colors of brilliance, and the colors contained in sunlight are far more extensive than the seven colors we see, but the light of other colors exceeds our visible range. The Sun is considered an "absolute blackbody" in physics, what is a blackbody? To put it simply, it is an object with strong radiation ability and weak reflection ability, and the sun is such an object, which is constantly radiating powerful electromagnetic waves outward every moment, but its own reflection ability is very weak. The electromagnetic wavelengths emitted by the sun cover a wide range, and the electromagnetic waves with longer wavelengths and the electromagnetic waves with shorter wavelengths contain relatively low energy, while the parts with relatively high radiation energy show seven colors of red, orange, yellow, green, blue, indigo and purple, which is what we usually call visible light.
Why does light with higher radiant energy happen to be light that is visible to the human eye? This is not a coincidence, but the result of the adaptation of organisms to sunlight over the course of their long evolution.
Since the sunlight visible to the human eye has seven colors, why does the sky only choose blue? The reason is scattering. We know that the reason why the sun can illuminate the earth is because the earth has an atmosphere, if there is no atmosphere, no matter how strong the sunlight is, it is only a bright spot in the distance, and watching the sun on the moon is this feeling. So why does the atmosphere scatter sunlight? This is because there are all kinds of tiny particles in the atmosphere, which are so small that they are less than one-tenth of the wavelength of the incident light, and then an optical phenomenon appears, which is called "Rayleigh scattering".
According to Rayleigh scattering, the scattering intensity of light is inversely proportional to the quaternion of the wavelength of the incident light, which means that the longer the wavelength, the weaker the scattering intensity.
Visible light is arranged from long to short according to the wavelength, red, orange, yellow, green, blue, indigo, purple, and among these seven colors, the light with strong scattering ability and large radiation energy is blue light, so naturally, under the action of Rayleigh scattering, the sky appears a blue. So is it because the sky is blue, so the sea is blue? Not.
The sea is composed of water, and the water is colorless and transparent, so the color of the sea depends on the absorption and reflection of light by the water. The absorption capacity of water for light with long wavelengths is very strong, so the light of the four colors of red, orange, yellow and green is basically absorbed by water molecules, and the light of the blue part is reflected back, so the sea we see is blue, and the deeper the place, the darker the color, and may even tend to purple.
So why is it that near the coastline, the color of the sea tends to be green, or even yellow?
In addition to the influence of optical properties, the color of the sea is also affected by the suspended solids in the ocean, due to the presence of a large number of suspended plants in the ocean, these plants have a strong absorption capacity for red and blue light, but they will reflect green light, so the places near the coastline will appear green, even yellow.
Speaking of why the sky is blue and why the sea is blue, we cannot fail to ask another question, that is, why is the sun red? These three questions can be said to be the classic triple questions of children for 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 a distinct red, and the sun at noon is actually bright white, so to answer this question, you can first look at the difference between the sun at noon 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 difference in angle.
The sun at noon is hanging high overhead, and the atmosphere penetrated by direct sunlight is relatively thin, so the light of seven colors will reach the ground and enter our eyes, and the seven colors are mixed together to show a bright white, so we think that the noon sun is bright white.
And the morning and evening sun, it is no longer located above our heads, but located in front of our oblique front, the angle is very large, from such a large angle of incidence, sunlight will penetrate a very thick atmosphere, so a large amount of sunlight will be consumed under the scattering action of the atmosphere, and we said earlier, the shorter the wavelength of light, the stronger its scattering ability, so the short wavelength of green light, blue light, indigo, purple light is scattered away, and the longest wavelength of red light due to the scattering ability is very weak, So it penetrates the atmosphere and comes into view, so the sun we see becomes red.