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.