WEBVTT
Kind: captions
Language: en-GB

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You probably already know that there are not
seven colours in the rainbow. Isaac Newton

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picked out seven colours because it was mystical,
it resonated with seven musical notes, and

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the seven days that he believed God took to
create the earth.

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You probably also already know that colours
are continuous. Unlike that projection artwork,

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there isn't one pure red, one pure orange.
In a real rainbow you see all the wavelengths

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of light in between.

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But what you might not know is that a rainbow
doesn't fade out at red and at violet. The

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EM wavelengths from the rainbow extend just
a little way into the invisible bits of the

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spectrum. Not far: water will absorb "proper"
infrared and ultraviolet. But you can see

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on what's called an absorption spectrum that
liquid water will let through -- and therefore

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refract into a rainbow -- wavelengths slightly
outside the ones we can see.

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If you take a photo of a rainbow with UV-sensitive
film, you will see another bright band below

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violet. And there are people out there who
don't have lenses in their eyes. They can

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still see, with strong glasses on, but the
UV filtering that the lens provides is gone.

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So they can see that extra band too. Over
there, in the National Gallery, are paintings

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by Claude Monet, who late in life had a cataract
operation that removed the lens of his eye

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-- and there's a strong argument that his
paintings after that were brighter and bluer,

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the result of the near-UV light he could then
see.

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There's one more question, then: why does
water happen to let through just the light

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we can see? If it was set up any differently,
water would be opaque to us, we might be blasted

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by hard UV light all the time, and life would
be... well, very different. The answer, of

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course, is that it isn't chance at all: over
hundreds of millions of years, our ancestors

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evolved those abilities. Our physics means
that water happens to be transparent at some

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wavelengths, so sensing those wavelengths
is an enormous advantage to life. The species

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we came from were much more likely to survive
with the ability to see those wavelengths.

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Imagine if you couldn't see through rain.
So those who couldn't see those wavelengths,

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well, they died more easily. Repeat for an
unimaginable number of generations, and here

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we are: creating our own rainbows.

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