Spotting rainbows is relatively easy, but chasing them and getting a decent composition with a compelling fore/background can be tricky. They can last for half an hour, but mostly they disappear within minutes. By understanding how they work, the possibilities of getting nice images will increase rapidly. Knowing that they build up best early morning or late afternoon, and the center of the bow is always opposite of the center of the sun. If you got a favorite spot which would look cool with a rainbow, you can plan your shot carefully if you know where the sun will set/rise. The most important tool for photography is a polarizer, which increases the colors of the rainbow to its maximum like you see it with the unaided eye, but which the sensor of the camera can´t record without the help of the polarizer.
The last couple of days we had some up and down weather, which resulted in beautiful rainbows. But only yesterday, the sun lasted long enough to get the images I was waiting for so long. While the patches of clouds has been wide enough, the rainbow has been visible for about 30 minutes; long enough for setting up and getting a proper composition.
An interesting site about optical phenomena: http://www.atoptics.co.uk/rainbows/primary.htm
A rainbow shows up behind St.Martin Cathedral, Landshut, Bavaria…
Altered image, done with Lightroom 3 only (Matt Koslowski Presets)
“Nature provides exceptions to every rule.”
(Margaret Fuller, American Critic, 1810-1850)