The formation of a rainbow

After the rain. It is true that rainbows sometimes appear in the sky, but rainbows can also appear around fountains or waterfalls in the sun; in summer, sometimes a section of rainbow can appear behind a sprinkler truck running on the street; spray the air with a sprayer Rainbows can also be formed…. Apparently, that kind of rainbow only exists. After the rain. The reason why rainbows appear is because the causes of rainbows are not fully understood. As long as you know that there are conditions in the air for rainbow formation, you will naturally know that it does not necessarily have to rain for a rainbow to appear.

In middle school physics class, there was one. Dispersion of light. Experiment: Take a prism and let a beam of white light pass through the slit and hit one side of the prism. After passing through the prism, its direction changes and a colored light band is formed on the white light screen. The order is red, orange, yellow, green, and blue. , indigo, purple seven colors. This is similar to the colors of the rainbow. But it is impossible for a prism to form a rainbow in the air. Why is this? This is because there are a lot of small water droplets floating in the air. When sunlight shines on these small water droplets, each small water droplet acts like a prism to decompose the white light into seven monochromatic lights, which plays a role in spectral dispersion of the sunlight.

How does sunlight produce spectral dispersion in small water droplets?

Sunlight enters small water droplets, that is, it enters the medium of water from the medium of air, and is refracted once. The refractive index of various monochromatic lights of white light is different. Purple light has the shortest wavelength and has the largest refractive index. Red light has the longest wavelength and has the smallest refractive index. The other colors of light are in between. Therefore, the light splits in the small water droplet. The light of various colors continues to propagate in the small water droplet at the same time. When it encounters another interface of the water droplet, it is reflected back, passes through the inside of the small water droplet again, and is refracted back to the air when it comes out. In this way, sunlight undergoes two refractions and one total reflection in small water droplets and is decomposed into seven monochromatic lights: red, orange, yellow, green, blue, indigo, and violet. When there are a lot of small water droplets in the air, the sunlight passes through these small water droplets, and after reflection and refraction, the light emitted is concentrated together, and a beautiful rainbow is formed in the sky.

Usually, most of what we see is a rainbow, with a viewing angle (from the ground to the top of the rainbow) of about 42°. Sometimes you can see another rainbow outside the rainbow with the exact opposite color sequence and darker color. This is called a secondary rainbow. The main rainbow is inner ultraviolet red, the secondary rainbow is inner infrared purple, and the secondary rainbow is also called neon. The neon and main rainbow are concentric arcs. The sky between them is darker, while the sky inside and outside the rainbow is brighter. Ni's viewing angle is approximately 51°. Its causes are basically the same as those of the main rainbow. It is formed by sunlight in small raindrops after two reflections and two refractions, that is, refraction-total reflection-total reflection-refraction. On the ground plane, the main rainbow and neon 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 main rainbow and neon. Only when the weather is clear and you look down in the airplane cabin, can you see the main rainbow and neon in full, that is, the complete circle.

If the angle of the sun is too large (for example, around noon), or too small (near sunrise or sunset), it is difficult for us to see a rainbow, because rainbows are sunlight reflected by small water droplets and enter our eyes. , so the rainbow always appears on the opposite side of the sun, therefore. Morning rainbows are seen in the west, and evening rainbows are seen in the east. . Its appearance mainly occurs in summer.

Why is the main rainbow internally ultraviolet red?

When we look at the rainbow, the colored light is reflected from the small water droplets at various angles. For a certain particle, only a certain particle can be reflected. One color of light enters our eyes, while other colored lights refracted from the same raindrop pass higher or lower than our eyes and are not seen by us. Specifically, among the light refracted by the small water droplets that can enter our eyes and are at the highest position, red light has the smallest refractive index and the smallest deflection angle, so it can enter our eyes. What we see Only red light and other colored lights pass over our heads due to their large refractive index and large deflection angle. Small water droplets that are slightly lower can only enter our eyes first and be seen by us because the deflection angle of the refracted light is larger than the red light and smaller than the other colors of light. Among the other colors of light, red light is on the low side, while yellow, green, blue, indigo and violet are on the high side and cannot be seen by us beyond our eyes. By analogy, for the light that enters our eyes and is refracted by the small water droplets at the lowest level, we can only see purple light, and the other colors of light slip away from under our eyes. In this way, the light refracted from adjacent small water droplets in the air forms a rainbow of inner ultraviolet and red.

The meteorological principle of the rainbow

The size of the water droplets in the air determines the color and width of the rainbow. The larger the raindrop, the narrower the rainbow band and the more vivid the color; the smaller the raindrop, the wider the rainbow band and the darker the color. When the raindrops are small enough, the light splitting and reflection are not obvious, and the rainbow disappears. This shows that the formation of rainbows is directly related to the existence, number, and size of raindrops in the air. In turn, rainbows are related to weather changes. For example, if the color of the rainbow changes from bright to dull, and the width changes from narrow to wide, it means that the raindrops in the air gradually become smaller. From this, we can speculate that the air may gradually become stable and the weather conditions will gradually become stable.

For thousands of years, the working people of our country have accumulated and spread many sky-viewing experiences related to rainbows in their long-term life and production practices, and have compiled them into proverbs in simple language.

These proverbs reflect the objective laws of weather changes and have become one of the basis for people to speculate on future weather changes. For example, "East rainbow sun, west rainbow rain" (or early rainbow rain, late rainbow clear) predicts future weather changes based on the appearance of rainbows. The rainbow is in the west, which means there are a lot of raindrops in the atmosphere in the west. As the weather system moves eastward, there will be rain locally. Western rainbows usually appear in the morning. If the rainbow is in the east, it means that there are raindrops in the atmosphere in the east, the weather system has moved over the local area, and the weather is about to clear up. Eastern rainbows usually appear in the evening. Another example: "When the rainbow is late and the sun is low, it rains early; when the rainbow is high and the sun is low, we wear a raincoat in the morning and evening; when the rainbow is high and the sun is low, the water covers the stream; when a broken rainbow is seen, the wind will follow it; when a broken rainbow is seen early, it will be seen by wind and rain; A rainbow eats a cloud with one finger, and a cloud eats a rainbow with a foot of the sky." These are all weather proverbs related to rainbows.

In addition to focusing on the synthesis of knowledge and abilities in this subject, physics teaching must also pay attention to the intersection and integration of knowledge with other subjects, so that knowledge comes from life and experiments, and returns to life and experiments, and back to Naturally, let students learn and apply it, and ultimately serve productive labor. "Rainbow's Principles of Meteorological Physics" is a reference example in physics teaching. It serves as a starting point to explore how physics teaching can tap into the intrinsic connections between physics and other subjects, adapt to new trends in comprehensive issues, and cultivate students' comprehensive qualities.