Why do rainbows have seven colors? Why not five colors, two colors, 1 color?

Rainbow is an optical phenomenon in meteorology. When the sun shines on the water droplets in mid-air, the light is refracted and reflected, forming an arched color spectrum in the sky. There are different opinions about which seven colors the rainbow is. China is the most common (from outside to inside): red, orange, yellow, green, cyan, blue and purple. The western saying is: red, orange, yellow, green, blue, indigo and purple are the names taken by scientist Newton after decomposing seven primary colors. In fact, as long as there are water droplets in the air and the sun shines behind the observer at a low angle, an observable rainbow phenomenon may occur. Rainbow usually appears in the afternoon, when the rain clears. At this time, the air is less dusty and full of water droplets, and one side of the sky is dark because of rain clouds. However, observers can see the sunlight without being covered by clouds above or behind them, so rainbows are easier to see. Another place where rainbows are often seen is near waterfalls. When the weather is clear, you can spray water or mist into the air with your back to the sun, or you can make rainbows artificially. Sunset is a rare phenomenon, which may appear in the night with strong moonlight. Because it is difficult for human vision to distinguish colors in the case of weak light at night, the night rainbow looks all white. The optical principle of rainbow is the dispersion and reflection caused by sunlight shining on spherical water droplets in the air. When sunlight enters the water drop, it will be incident at different angles at the same time and reflected at different angles in the water drop. The reflection of 40 to 42 degrees is the strongest, which produces the rainbow we see. When this reflection occurs, sunlight enters the water drop, refracts once, then reflects on the back of the water drop, and finally refracts again when it leaves the water drop. Because water has a dispersive effect on light, the refractive index of light with different wavelengths is different, and the refractive angle of blue light is greater than that of red light. Because light is reflected in water droplets, the spectrum seen by the observer is reversed, with red light at the top and other colors at the bottom. Double rainbow, the outer ring is neon, and the inner ring is rainbow. After the rain, the double rainbow over Nuremberg will often see two rainbows appear at the same time, and a concentric but darker secondary rainbow (also known as neon) will appear outside the usual rainbow. The secondary rainbow is formed by two reflections of sunlight in water droplets. The strongest reflection angle of the two reflections occurs at 50 to 53, so the position of the secondary rainbow is outside the main rainbow. Because there are two reflections, the color sequence of the auxiliary rainbow is opposite to that of the main rainbow, with the outer side being blue and the inner side being red. In fact, the auxiliary rainbow must follow the main rainbow, but because of its low light intensity, it is sometimes invisible to the naked eye (the rainbow does not actually appear in a specific position in mid-air. It is an optical phenomenon seen by the observer, and the position of the rainbow will change with the observer. When an observer sees a rainbow, its position must be in the opposite direction of the sun. The center inside the rainbow arch is actually an enlarged image of the sun reflected by water droplets. So the sky inside the rainbow is brighter than the sky outside the rainbow. The center of the rainbow arch is just the direction of the observer's head shadow, and the rainbow itself is 40 to 42 degrees above the line between the observer's head shadow and his eyes. Therefore, when the sun is higher than 42 degrees in the sky, the rainbow will be below the horizon and invisible. This is why rainbows rarely appear at noon. The rainbow extends from one end to the other. With an ordinary 35mm camera, you need a wide-angle lens with a focal length below 19mm to capture the whole rainbow in a single frame. If you are on an airplane, you will see that the rainbow will be a complete circle instead of an arch, and the center of the circular rainbow is the direction of the airplane. Double rainbow over Scotland 1307, it was suggested in Europe that the rainbow was caused by the refraction and reflection of sunlight by water droplets. Descartes found in 1637 that the size of water droplets does not affect the refraction of light. He experimented by injecting water into the glass ball and got the refractive index of water to light. He mathematically proved that the main rainbow is caused by reflection at the water point, while the auxiliary rainbow is caused by two reflections. He calculated the angle of the rainbow accurately, but failed to explain its colorful colors. Later, Newton discovered all the optical principles of rainbow formation after scattering sunlight into color with a glass rhombus.