Why are rainbows arcs or semi circles?
A. Rainbows are usually only partial – part of – the cirlce around the anti solar point (shadow of your head) on which rainbows are seen. They can not be square or straight, because of the way rainbows are formed. Read about how a rainbow is formed to understand this better. Q. Can a rainbow be a complete circle? A. Yes. It is difficult to produce the conditions for this to occur. It isn’t really possible to have a complete circle from a naturally occurring rainbow as seen from someone standing on the ground looking at a rain shower. However in an aircraft you could possible be able to look down on water drops in the atmosphere and see a rainbow. In reality you are more likely to be looking at cloud or fog where the water drops are very small. When the water drops get very small the wave like nature of light starts to dominate which will wash out the colours in a rainbow until the point the drops are too small and a rainbow will not been seen at all. So a complete circular rainbow seen
