Why is a rainbow an arc instead of each rain drop seperating light to form tiny straight rainbows?
A rainbow is formed because light is bent as it passes through the rain drop. This only happens at a specific angle. Which means that in order to see the rainbow, the rain drops must be a certain angle from you. If you map out all of the points that are at this angle, you will see that if forms a circle around you. Of course you don’t see the bottom part of the circle since that part would have to come from underground. This is also the reason that rainbows appear to move as you approach them. What is change is the portion of the sky that is that angle from you.
