Just wondering... what causes a rainbow to curve?

According to Stephen Colbert, rainbows are God's way of frowning on gay pride parades

 

Rainbows are an effect of multiple water droplets acting as a prism. This is why you see rainbows at waterfalls. You can make your own rainbow by spraying water on a sunny day. The rainbow's center would be along a line from the sun to your head to the rainbow's center (toward the shadow of your head), and the rainbow itself would be at a radius around that, if there is water droplets there (at least I think that is right).

The light that can get refracted back to your eye must have a constant angle with respect to the light source. That geometry describes a cone, whose base is a circle.

A rainbow seen from a plane can be a complete circle.

 

To see lots of information and really pretty photos take a look at this link.

 

http://www.atoptics.co.uk/bows.htm