The color of the rainbow is formed by the refraction and total reflection of water droplets, so the rainbow must appear in the direction of the extension line between the sun and you. In fact, the rainbow should be round in someone's eyes, but because the other half is blocked by the earth, you can only see half, so it is a semicircle. The extension line connecting the sun and your eyes passes through the center of the circle.

If you turn your back to the sun on a sunny day, and then spray the fog with the sprayer in front of you, then you can see a round rainbow. If it is around noon, the effect will be more obvious. You can often see round rainbows on airplanes.

In fact, when you look at a rainbow, all the water droplets in the canopy of the rainbow can reflect light, but you can only see the light reflected from an arc area.