Yesterday’s rainbow prompted me to look again at how rainbows are produced.
Spherical raindrops act as a prism to split the white light from the sun into the different coloured elements with red refracted slightly less than violet which is at the other end of the 
visible spectrum. The red bow of the rainbow is at an angle of about 40 degrees from the direction of the incoming sunlight. The violet bow is about 42 degrees. The secondary rainbow (just visible on the photos outside the main one) is reversed and at 50 to 53 degrees.
There are some good explanations of the physics on the web – some references below. Neither is very clear on why the reflection/refraction is optimum at the 40/42 degree angles: I think the internal reflection from the back of the raindrop is at a maximum at this angle but the applet that allows you to vary the incident angle of the incoming sunlight doesn’t make this clear. I’ll have to do more research.
http://science.howstuffworks.com/rainbow.htm
While looking at the last of the above links, I clicked on the home page – the (US) National Center [sorry!] for Atmospheric Research. The building, near Bounder, Colorado, a reflection of the surrounding ‘Flatiron’ rocks, is designed by IM Pei. Pei designed the Louvre pyramids in Paris and a similar design that I worked in for a short while at IBM Somers. Although ‘modern’ (it was built in the early 1960s) the NCAR building seems to fit in well. Why can’t we be braver about our buildings?
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