Why Rainbows are semi-circular?

Question

why are rainbows semi-circular, why cant it form in other geometrical shapes like circle, square. etc

Answer 1

You need a few things to see a rainbow. The main things are rain, you need that rain to be illuminated by bright sunshine, and the rain must be in the right position relative to you. A primary rainbow is always somewhere on an arc 42 degrees around the shadow of your head (called the anti solar point). So the brightly illuminated rain must be in this direction away from you to see a rainbow.
Rainbows always appear round because they are formed at a constant angle around the anti solar point (the shadow of your head). The shape of something described by an arc a constant distance away from a point is a circle, which is called round.

Answer 2

Actually rainbows are a complete circle. Sometime you can see the whole rainbow when you are flying.