The visible light of the sun-red, orange, yellow, green, blue, indigo and purple-has different wavelengths. When they irradiate these water droplets in the air, the light of each color is refracted by the water droplets to varying degrees, and then decomposed into seven-color spectra.
The visibility of rainbow is closely related to the size of water droplets. The larger the volume of water droplets, the more obvious and brighter the rainbow we can see with the naked eye; The smaller the water drop, the less obvious it is.
Rainbow is the cause of arc.
When a light beam touches a water drop, there are usually two situations: one is that the light may pass through it without hindrance; The other is that it may hit the front of water droplets. When entering, it bends inside the water drop, then reflects back from the back end of the water drop, then leaves from the front end of the water drop and then refracts. The degree to which light bends when it passes through a water drop depends on the wavelength (i.e. color) of the light.
Among the seven colors of light, each color has a corresponding wavelength, and different wavelengths of light will have different curvatures when passing through water droplets. Among them, red light has the largest curvature, followed by orange light and yellow light, and so on, and purple light has the smallest curvature. In addition, the reason why the rainbow is curved is also related to the fact that the earth itself is a sphere and is covered by a thick atmosphere.
Refer to the above? Popular Science China —— Three Why's about Rainbow