Rainbow is very wonderful . It is formed by many prism-like rain droplets..but not clear why big size curve?

Answer 1

In order for you to see color reflected back at you from a raindrop, the angle from the sun to the raindrop to your eye has to be just right. All the raindrops at that exact angle would form a circle, but the bottom of the circle gets cut off where the raindrops hit the ground, so you just see an arc of color.