Why are rainbows in an arc-shaped?

118th Discovery Posting

I am sure that many of you have seen a rainbow in the skies above you before. But have you ever wondered why are rainbows shaped in an arc? Personally, I am not sure of it myself and I was made curious about the answer by one of my participants in a recently guided inter tidal walk who asked his son this question when they saw a rainbow over at Big Sisters' island not long ago. As a result of this curiosity, I decided to do some simple research on the internet and do up this blog post.
The first thing to note is that rainbows are caused sunlight passing through raindrops and by refraction, which is the bending of light as it passes from one medium to another. I would not go through the exact science of how to explain this as I have found websites that does this explanation better than I can. You can find the websites below.

It is interesting to note that a rainbow is really circle in shape than the arc shaped or semi circle shape we usually see. The reason why we don't see the whole circle is because the earth blocks our view. And the closer the sun is to the horizon, the more of the circle we see. So this means that the best time to see a longest rainbow would be either during early morning after sunrise and evening time before sunset. =D
Great websites that explains how a rainbow is formed and why is it in a arc.
a) http://science.howstuffworks.com/rainbow.htm
b) http://www.scientificamerican.com/article.cfm?id=why-do-rainbows-form-inst

Side note:
Rainbow Connection by Kermit the Frog. Enjoy! =D