You probably already know that there are not seven colours in the rainbow. Isaac Newton
picked out seven colours because it was mystical, it resonated with seven musical notes, and
the seven days that he believed God took to create the earth.
You probably also already know that colours are continuous. Unlike that projection artwork,
there isn't one pure red, one pure orange. In a real rainbow you see all the wavelengths
of light in between.
But what you might not know is that a rainbow doesn't fade out at red and at violet. The
EM wavelengths from the rainbow extend just a little way into the invisible bits of the
spectrum. Not far: water will absorb "proper" infrared and ultraviolet. But you can see
on what's called an absorption spectrum that liquid water will let through -- and therefore
refract into a rainbow -- wavelengths slightly outside the ones we can see.
If you take a photo of a rainbow with UV-sensitive film, you will see another bright band below
violet. And there are people out there who don't have lenses in their eyes. They can
still see, with strong glasses on, but the UV filtering that the lens provides is gone.
So they can see that extra band too. Over there, in the National Gallery, are paintings
by Claude Monet, who late in life had a cataract operation that removed the lens of his eye
-- and there's a strong argument that his paintings after that were brighter and bluer,
the result of the near-UV light he could then see.
There's one more question, then: why does water happen to let through just the light
we can see? If it was set up any differently, water would be opaque to us, we might be blasted
by hard UV light all the time, and life would be... well, very different. The answer, of
course, is that it isn't chance at all: over hundreds of millions of years, our ancestors
evolved those abilities. Our physics means that water happens to be transparent at some
wavelengths, so sensing those wavelengths is an enormous advantage to life. The species
we came from were much more likely to survive with the ability to see those wavelengths.
Imagine if you couldn't see through rain. So those who couldn't see those wavelengths,
well, they died more easily. Repeat for an unimaginable number of generations, and here
we are: creating our own rainbows.
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