How is the rainbow formed?

Rainbow is a natural light phenomenon that people often see. Whenever colorful rainbows hang in the sky, people can't help rushing to this beautiful nature. In ancient times, some people said that it was the lonely Chang 'e singing and dancing colorful silk in the clouds; Some people say that it is a colorful bridge built by fairies in the clouds to peep at the world. Whether it is colored silk or colored bridge, it is just a magical legend. And what is the rainbow in reality? How is it formed?

When it comes to the formation of rainbows, people often think of it. After rain. Be with each other. Many people think that only. After rain. There is a rainbow. Actually, this view is not comprehensive. . After rain. It is true that rainbows sometimes appear in the sky, 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. Obviously, that kind of rainbow only exists in. After rain. The reason for the rainbow has not been fully understood. As long as we know that it is conditional to see rainbows in the air, we naturally know that rainbows don't have to rain to appear.

There is a teacher in the middle school physics class. Scattering of light. Experiment: Take a prism and let a beam of white light pass through the slit and shine on one side of the prism. After passing through the prism, the advancing direction 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. But it is impossible to have a prism in the air to form a rainbow. Why is this? 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.

How does sunlight produce spectral dispersion in small water droplets?

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.

Usually, most of what we see is a rainbow, and the viewing angle (from the ground to the top of the rainbow) is about 42. 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. It is formed by two reflections and two refractions of sunlight in small raindrops, namely refraction-total reflection-total reflection-refraction. 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.

If the angle of the sun is too big (for example, around noon) or too small (rising or setting recently), we can't easily see rainbows, and because rainbows enter our eyes through the reflection of sunlight by small water droplets, rainbows always appear opposite the sun. The morning rainbow is in the west and the evening rainbow is in the east. . It appears mainly in summer.

Why is the main rainbow purple?

When we look at a rainbow, colored light is reflected from water droplets at various angles. For a particle, only one color of light can enter our eyes, while the light of other colors refracted by the same raindrop passes through our eyes high or low without being seen by us. Specifically, among the light that can enter our eyes and be refracted by water droplets at the highest position, red light can enter our eyes because it has the smallest refractive index and the smallest deflection angle. We only see red light, and other colors of light pass over our heads because of their large refractive index and large deflection angle. A slightly lower water drop can only be seen by orange light with deflection angle greater than red light and less than other colors under refracted light. Among other colors, red light is low, yellow, green, blue, indigo and purple are high, which are beyond our eyes. By analogy, the light that enters our eyes and is refracted by the lowest water drop can only see purple light, and the rest of the colored light slips away from our noses. In this way, the light refracted by adjacent water droplets in the air forms a rainbow, which is ultraviolet and red.

Meteorological principle of rainbow

The size of water droplets in the air determines the color and width of the rainbow. The bigger the raindrops, the narrower the rainbow band and the brighter the color. The smaller the raindrops, the wider the rainbow band and the darker the color. When raindrops are small enough, the light splitting and reflection are not obvious, and the rainbow disappears. This shows that the formation of rainbow is directly related to the existence, quantity and size of raindrops in the air, and in turn, rainbow is related to weather changes. For example, the color of the rainbow changes from bright to dark, and the width changes from narrow to wide, indicating that the raindrops in the air change from big to small. It can be speculated that the air may gradually stabilize and the weather conditions will gradually stabilize.

For thousands of years, the working people in our country have accumulated and circulated many experiences related to observing the rainbow and the sky in their long-term life and production practice, and compiled proverbs in concise language. These proverbs reflect the objective laws of weather changes and become one of the bases for people to speculate on future weather changes. For example, "east rainbow sunrise, west rainbow rain" (or early rainbow rain, late rainbow sunny), according to the appearance of rainbow to predict future weather changes. The rainbow is in the west, indicating that there are a lot of raindrops in the atmosphere in the west, which move eastward with the weather system; It will rain locally; Xihong often appears in the morning. The rainbow is in the east, indicating that there are raindrops in the eastern atmosphere, and the weather system has moved over the local area, and the weather is about to clear up; Donghong mostly appears at night. Another example is: "rainbow in the evening and rainbow rain in the morning;" The rainbow is high and the sun is low, and you will wear hemp fiber in the morning and evening; The rainbow is high and the sun is low, and the flood does not cross the stream; Look at the broken rainbow and see it with the wind; If you break the rainbow, you will see it soon. The rainbow eats the next finger of the cloud, and the cloud eats the next foot of the rainbow. Are all weather proverbs related to rainbows.

Physics teaching should not only pay attention to the synthesis of knowledge and ability in this discipline, but also pay attention to the intersection and integration with other disciplines, so that knowledge can come from life and experiment, return to life and experiment, return to nature, make learning flexible, and finally serve productive labor. The Principle of Meteorological Physics of Rainbow is a reference example in physics teaching, which aims to attract more attention and explore how physics teaching can tap the internal relationship between physics and other disciplines, adapt to the new trend of comprehensive problems and cultivate students' comprehensive quality.

Semicircular rainbows often appear in the sky after the rain clears. It is a light band composed of seven colors: red, orange, yellow, green, blue, indigo and purple. We call it a rainbow.

The ancients in our country paid great attention to observing natural phenomena and had a scientific explanation for the rainbow in the sky a long time ago. Shen Kuo quoted Sun Yanguang as saying in Meng Qian Bi Tan: "Rainbow, rain shadow is in the middle of the sky, and rain is in the sun, so there is it." In Xuanzhenzi written by Zhang in the Tang Dynasty, it said: "Water sprays on the back of the sun to form a rainbow." It can be seen that the rainbow is formed by sunlight shining on water droplets in the air.

We know that when sunlight passes through a prism, it will disperse, and the original white light will be decomposed into seven colors: red, orange, yellow, green, blue, indigo and purple. Before or after the rain, there are many tiny water droplets suspended in the sky, which act as prisms. When the sun shines on water droplets, as shown in figure 1, it will happen once when they enter water droplets. Then, when the water drops shoot out, they are refracted again. These two refractions and reflections make colored light with different frequencies move in different directions, so dispersion occurs. Because sunlight is parallel light, only one direction emits the strongest light, and this direction is the direction in which we see the rainbow. The angle between this direction and the direction of incident sunlight is about 42 for red light, 40 for violet light and 40 for other colors. As shown in Figure 2, because there are many water drops in the sky, the rainbow can only be seen from the direction of the above angle, so the rainbow seen standing on the ground is semi-circular with ultraviolet red inside, as shown in Figure 3.

So why do rainbows often appear after rain in summer, but not in winter? Because there are often thunderstorms or showers in summer, the scope of these rains is not large. It often rains here, and the sky there is still sunny. After the rain, there are still many small drops of water hanging in the sky. In winter, the weather is generally cold, the air is dry, and there are fewer opportunities for rain, so there are fewer showers. Most of it is snowfall, and snowfall does not form rainbows, so rainbows rarely appear in winter.

When sunlight travels in the sky and meets tiny water droplets in the sky, the light will be refracted into the water droplets.

Due to the different bending degrees of different colors of light, different colors of light in water droplets are separated.

When the light meets the boundary between water droplets and air for the second time, most of the light will be refracted again soon.

However, when a small part of the light reflected once in the water droplet meets the boundary between the water droplet and the air for the third time,

Part of the refracted light will form a rainbow.

When sunlight penetrates rain particles, a rainbow is formed. Originally, light travels in a straight line, but once it enters the water, it also has the property of refraction. So sunlight will be refracted when it passes through rain particles. At this time, because the angle of light refraction varies with different colors, seven colors will refract at different angles. So the seven colors will be arranged beautifully. This is the principle of forming a rainbow. Because the rainbow appears in the sky opposite the sun, if you want to see the rainbow after the rain, you should turn your back on the sun.