Looking at the rainbow from the sky, is this really the case?

Recently, various social networking sites are circulating a beautiful picture? Looking down from the sky, the rainbow you see is like this:

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Looking down from the sky, the rainbow you see is like this.

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How beautiful! Even Tadpole Jun can't help feeling! But is it true? Let's make up the physics class in middle school.

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Rainbow is a natural light phenomenon that people often see. How is it formed? When it comes to the formation of rainbow, people often associate it with rain. Many people think that rainbows can only appear after rain. Actually, this view is not comprehensive. It is true that rainbows sometimes appear in the sky after rain, but rainbows also appear around fountains or waterfalls in the sun. In summer, sometimes there will be a rainbow behind the sprinkler running in the street; Spraying in the air with a sprayer can also form a rainbow.

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Most people have seen this ordinary rainbow. They are so common that people can see them even when they water plants in the garden.

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There is a light dispersion experiment in middle school physics class: take a prism, let a beam of white light pass through a slit and hit one side of the prism. After passing through the prism, the direction of progress changes, and a colored light band is formed on the white screen. The order is red, orange, yellow, green, blue, indigo and purple. This is very similar to the color of a rainbow.

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But it is impossible to have a prism in the air to form a rainbow. Why is this?

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This is because there are a lot of small water droplets floating in the air. When the sun shines on these small water droplets, one by one, like a prism, decomposes the white light into seven monochromatic lights, which have dispersion effect on the sun.

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How does sunlight produce spectral dispersion in small water droplets?

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Sunlight enters water droplets, that is, it enters water from the medium of air and undergoes a refraction. Due to the different refractive indexes of various monochromatic lights that make up white light, violet light has the shortest wavelength and the largest refractive index, red light has the longest refractive index and the smallest refractive index, and other colored lights are in between. Therefore, the light is split in the water drop, and the light of various colors continues to spread in the water drop at the same time, and is reflected back when it meets another interface of the water drop, passes through the inside of the water drop again, and refracts back into the air when it comes out. In this way, the sunlight in the water drops undergoes twice refraction and once total reflection, and is decomposed into seven monochromatic lights: red, orange, yellow, green, blue, indigo and purple. When there are a large number of water droplets in the air, sunlight passes through these water droplets, and after reflection and refraction, the emitted light gathers together to form a beautiful rainbow in the sky.

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No matter what the weather is like, waterfalls will reverse the mist generated by flowing streams into the air, and the saturation of the atmosphere will continue to move. This makes Waterfalls an excellent photography partner? The appearance of rainbow.

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On sunny days, the wind stirs up waves in the ocean or lake. When the water mist and humidity in the air reach saturation, it will form the variation of waterfall rainbow? Spray a rainbow? .

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Usually what we see is mostly a rainbow with a viewing angle of about 42 (from the ground to the top of the rainbow)? . Sometimes you can see another rainbow with the opposite color order and deeper color outside the rainbow. This rainbow is called the second rainbow. The main rainbow is internal ultraviolet red, the auxiliary rainbow is internal infrared purple, and the auxiliary rainbow is also called neon. Neon and main rainbow are concentric arcs, and the sky between them is dark, while the sky inside and outside the rainbow is bright. The viewing angle of neon is about 5 1? . Its origin is basically the same as the main rainbow. Is the sunlight reflected twice and refracted twice in small raindrops, that is, refraction? Total reflection? Total reflection? Formed by refraction.

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Secondary rainbows often appear with main rainbows. They are usually thinner and darker than the main rainbow. What is the part between the main rainbow and the secondary rainbow that is darker than the surrounding sky? Alexander Bert? (That is? Dark zone? )。

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Is the key part here? On the ground level, the main rainbows and neon lights we see are semicircular because their lower parts are covered by the ground. If you stand on the top of a high mountain, you can see most of the major rainbows and neon lights. Only when the weather is clear, looking down in the aircraft cabin, can we see the whole picture of the main rainbow and neon, that is, a complete circle.

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Although it is often difficult to see a complete rainbow because of the obstruction of the ground, in fact, most of the visible rainbows are circular arcs with perfect circles (according to Descartes, the exact radius of an arc is 42 degrees).

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If the angle of the sun is too big (for example, around noon) or too small (the recently rising or setting sun), it is difficult for us to see rainbows, and because rainbows are reflected by sunlight through small drops, they always appear opposite to the sun, so we see rainbows in the morning in the west and rainbows in the evening in the east. It appears mainly in summer.

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There are some more beautiful rainbows in P.S., which not everyone has a chance to see.

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Red rainbows are usually visible at sunrise or sunset, when the thickness of the earth's atmosphere filters out blue light, leaving more red or orange light reflected or refracted by water droplets. In this way, the edge of the rainbow becomes redder and redder.

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Fire rainbow is actually not a rainbow, and it has nothing to do with fire. ? A horizontal arc around the ground? Is the real name of this exquisite optical effect.

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Compared with rainbows, fog rainbows are very rare, because this phenomenon will only appear under certain conditions. On the one hand, the light source must be behind the observer and below the horizon. At the same time, the fog behind the observer must be very thin, so that the sun can shine on the thicker fog in front.

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Like Gengetsu, the moon rainbow is equivalent to the rainbow made by the moon. Moon rainbow is also difficult to observe, because it needs to go through a storm. Ideally, it is a bright full moon, which is not locked by clouds.

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With the emergence of powered aircraft and aerial photography technology, people have the opportunity to witness the spectacular circular rainbow. So what we see on the plane or on the mountain should be a rainbow ring, not the effect of adding polarized lenses.

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This photo was taken by the parachutist from mid-air, and you can see the complete circular rainbow. However, the rainbow seen on the plane is not always a complete circular rainbow. Skydivers are low in height and close to the ground, and the water droplets in the fog are large, which can form colorful rainbows. However, the plane is located at high altitude, and the diameter of cloud droplets at high altitude is usually very small, so Bai Hong is usually seen. Moreover, the standard circular rainbow can only be seen when the sun is at the zenith, and when the height angle of the sun is greater than 42? Less than 90? When I saw it, I saw an oval rainbow; When the altitude angle is less than 42? What you see is a hyperbola.

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refer to

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1、? Knowledge of Modern Physics, No.3, 2004, Kang Liangxi.

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2、? Some pictures are from Shell Net and chinese national geography.

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Author: Tadpole Jun comprehensive report