What a wonderful sight - I saw a double rainbow for the first time today. What made it even more joyous was I saw it with my daughter who is seven years old and my mother-in-law who is eighty years old. And everyone of us, irrespective of the age found it fascinating. Wordsworth captures the emotion flawlessly:
My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
Or let me die!
The Child is father of the Man;
And I could wish my days to be
Bound each to each by natural piety.
Rainbows are fascinating even when you know the phenomena of refraction and reflection that cause it. And how did Newton decide there were only seven colours in the spectrum. Of course the colours are indistinct and diffused and it is a continuous spectrum. As to how Newton decided on the number 7 there are several surmises ranging from how he drew an analogy with the seven notes in music to seven days in a week and even that he considered seven to be a 'spiritual' number. But the fallout of it is whenever we see a rainbow we try to identify the seven colours though it is generally very difficult to distinguish the violet - blue - indigo bands.
When Newton explained the phenomenon of rainbow, John Keats famously lamented the 'unweaving of rainbow' thus:
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, as it erewhile made
The tender-person’d Lamia melt into a shade.
My most favorite of Richard Dawkins' books is 'Unweaving the Rainbow: Science, Delusion and the Appetite for wonder' which wonderfully seeks to counter Keats' view and instead argues that the poetry of the rainbow was not destroyed by Newton; instead it seeks to enhance our appreciation of Nature in all its glory.