Rainbow Colours
Rainbows rarely come brighter or more colourful than this. Sándor Pócsai captured it at Davod, Hungary. The bow is made by nearby large raindrops. ©Sándor Pócsai, shown with permission

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Red - orange - yellow - green - blue - indigo - violet are the traditional rainbow colours. In fact, there is an infinite gradation of hues.

All rainbow hues are mixtures of the other infinity of pure spectral colours.

At left is a computed rainbow made by large drops where diffraction effects might be ignored. A few hundred overlapping pure colours were used - short of infinite but enough to give a reasonable result. Just 13 are shown as examples at left.

In the rainbow each pure colour would have a sharp edge at the bow's rim at right and tail off gradually in brightness towards the bow centre at left. "Would have" because the sun is a 0.5° diameter disk and blurs the intensity profile.

Take the green part of the bow for instance. Oranges, yellows and reds all add their bit to the final hue. Each part of the rainbow has colours of longer wavelength mixed in .

There is much more of rainbows to unweave. Poet Keats is oft quoted ...
  "Do not all charms fly
At the mere touch of cold philosophy?
There was an awful rainbow once in heaven:
We know her woof, her texture; she is given
In the dull catalogue of common things.
Philosophy will clip an Angel's wings,
Conquer all mysteries by rule and line,
Empty the haunted air, and gnomed mine--
Unweave a rainbow..

Oh! If only the poet had learned a little natural philosophy. He would have known that its touch gives to the catalogue of common things magic and mysteries. It fills our imagination and lifts it on powerful beating wings.