<h1 class="pgc-h-arrow-right" > what color is the sky? </h1>
This is a question that curious children often ask. Whenever this happens, the adults will tell them with great certainty: "The sky is blue!" ”
Indeed, in clear weather, when we look up at the sky, most of the time it always exhibits a pleasant blue color. The blue sky is dotted with white clouds, and only when it is cloudy or late at night can we see a blue sky.
Blue sky and green space, often refreshing
<h1 class="pgc-h-arrow-right" > why is the sky blue? </h1>
Faced with the second question posed by curious children, many people do not know how to answer. You might say, it's not that simple, because the sea is blue! The sea reflects the blue into the sky, so the sky turns blue. It is also said that the sky appears blue because oxygen is blue. In fact, neither of these answers is correct.
Blue sky
<h1 class="pgc-h-arrow-right" > the color of the sun</h1>
When the clouds are thick, we can't see the blue sky; in the middle of the night, the sky looks black. This is all because we can't see sunlight, and we can only see the clear blue sky when sunlight passes through the atmosphere. So the color of the sky is closely related to the sunlight.
So, what color does sunlight do?
A common claim is that sunlight is yellow or orange.
What color is the sun?
Correct answer: In our view as humans, sunlight is white.
At the same time, scientists will tell you that the sun itself does not emit white light, and the white of sunlight is actually the result of a mixture of all the visible light frequencies emitted by the sun. If you let sunlight pass through a triangular prism, the white will scatter, forming a colorful band of light that is orange, yellow, green, and blue.
When sunlight passes through a transparent medium, such as glass, water or air, light of different frequencies and wavelengths will be refracted to varying degrees, so we can distinguish the light waves that were originally mixed together and see part of the original color of the sunlight.
White light refracts as it passes through the prism
Why some but not all colors?
Sunlight is an electromagnetic wave emitted by the sun, and its wavelength range is very wide, in addition to the part that is visible to our naked eye, sunlight also includes γ rays, X-rays, ultraviolet rays, infrared rays, microwaves and longer wavelength radio waves. Of all these spectrums, the visible light waves that the human eye can perceive are only a small fraction of them.
Visible light makes up only a small fraction of the spectrum
<h1 class="pgc-h-arrow-right" >what happens when sunlight passes through the atmosphere? </h1>
The Earth's atmosphere is filled with various gas molecules, of which about 78% are nitrogen and about 21% are oxygen. When the light hits these gas molecules, electromagnetic waves will cause the charged particles in the gas molecules to oscillate, and the oscillating charge will produce electromagnetic radiation of the same frequency as the incident sunlight, but the electromagnetic radiation emitted by these gas molecules will be scattered in all different directions, which is what we often call the light scattering phenomenon.
Air molecules have a difference in their ability to scatter light. In the spectrum of visible light, the violet and blue parts have shorter wavelengths and faster frequencies than red light waves, and blue light can make charged particles in the air molecules oscillate at a higher frequency, resulting in more scattered light, so blue light scatters more intensely than red light. The intensity of light waves scattering in the air is proportional to the quaternion of the light frequency.
Scattering of sunlight in the atmosphere
For light waves with longer wavelengths, such as yellow and red light, they are more likely to pass through the atmosphere, while microwaves and radio waves can travel through the air almost unobstructed due to their long wavelengths.
Blue and purple light waves are more than 9 times more likely to be scattered in the atmosphere than red light, and they are more likely to be scattered, so when the sun appears high in the sky, the blue sky we see is the result of air scattering blue, green, and a small amount of other colors of sunlight.
Sunlight passes through the atmosphere
You might ask, since light with short wavelengths is more likely to be scattered by air, why can't we see purple light? This is because purple light is already scattered first at high altitudes, and there are very few purple light waves that can reach the ground; at the same time, the light-feeling cells of our eye's retina are not sensitive to purple light, and they are more easily understood as deeper blue.
<h1 class="pgc-h-arrow-right" > sky is always blue? </h1>
Of course not. When the sun is low in the morning and at dusk, we see the sky near the horizon taking on a brilliant orange-red color. At this time, the sunlight needs to pass through more atmosphere before reaching our eyeballs, so we see bright light. Those light with shorter wavelengths could not penetrate such thick air, and they were scattered high in the air, so the sky above our heads was indigo blue.
In the morning, the sky near the horizon is redder
Curious, you may ask, our Earth is called the Blue Planet, and when viewed from space, does it appear blue because the atmosphere reflects blue light?
Why is the Earth blue?
In fact, if you look at the Earth from space, the atmosphere can only reflect very little blue light, and the blue planet we see is more the result of the ocean reflecting the sun's rays. More than 70% of the Earth's area is covered with oceans, and blue light waves in sunlight travel the longest distance in seawater, while other colors of light are absorbed by seawater, so we see that the sea is blue. The sea reflects blue light into space, and we see the blue Earth. If there is no sea on the surface of the Earth, then it is not a blue planet.
The sea is blue, the earth is blue
<h1 class="pgc-h-arrow-right" > summary:</h1>
The reason why our sky is blue is first related to the spectral composition of sunlight, white sunlight is actually composed of many different wavelengths of different colors of light, when these rays pass through the Earth's atmosphere, different degrees of scattering phenomenon occurs.
Earth's air molecules have a higher scattering rate of blue and violet light with shorter wavelengths, other colors of visible light are easier to pass through, and the human eye is not sensitive to purple, so when sunlight hits the atmosphere, we see the sky appear blue.
Do you get it?