We had a very wet holiday on Anglesey over Easter but I did manage to catch up with a double rainbow. I read about these years ago but pulled up short a year or so ago when I finally noticed that the colours are reversed on the outer rainbow. Enlarge this picture and take a good look at it.The colours of a rainbow are caused by total internal reflection of the light in a raindrop and the subsequent refraction as it leaves the front of the drop. For a double rainbow, the light reflects twice on the inside before exiting.
See http://en.wikipedia.org/wiki/Rainbow
