Nature 2015 October

Unweaving the Rainbow – Holding up a prism to the wonders of the natural world

Without realising it we tend to spend more of our time viewing the world through a prism. In other words there is a tendency to focus on the assemblage of many individual things rather than taking the step back to consider the whole picture. Occasionally though we are struck by the wonder of nature. However many times we spy a rainbow we still stop at least momentarily and stare, taking in and enjoy the whole spectacle. What is more we also take pleasure in sharing this experience with others. And this must have been a human activity from time immemorial.

Alongside all this wonder of rainbows poets, philosophers and scientists sought to describe, apply purpose to, and provide explanations for this phenomenon. Many such explanations were put forward, starting with Aristotle in the Third Century BC. The Persians and Chinese in the Middle Ages both rightly associated the multi-coloured display with raindrops. It was Theodoric of Freiberg in the 14th century who first discovered the prismatic qualities of ‘moist air’ in producing rainbows. However, it was left, first to Descartes in the mid 1600s, and then Sir Isaac Newton around 1700 to provide the first fully scientific explanation for this wonder of the natural world.

In short, Newton concluded that the ‘seven colours of the rainbow’ were the product of white light being split out into its constituent frequencies of the visible spectrum. The splitting out of each element of the colour spectrum was down to light being refracted to a different degree as they pass through drops of water. In brief, red light is refracted at a lesser angle than is blue light. The human eye is left to capture just a reflection of this phenomena. For scientific investigation, equally important to the discovery of light’s physical properties was the elegance of Newton’s scientific experimental method which was on a par with the importance of his discovery. Starting with the already established theories about light, through elegant experimental techniques, and by observation and measurement, at last Newton was able to deduce a robust scientific explanation. Newton started by employing two prisms. The first prism split white light into its constituent colours and the second focussed on only the red light showing it was unchanged when passing through it. It was the serendipity of having access to the first truly accurate prismatic lenses, only recently developed for the latest telescopes and microscopes, which enabled Newton to design this elegant experiment. The recombination of the red through to blue colours previously split out by prisms back into white light, provided the answer to the puzzle of the rainbow.

Despite this discovery Newton was unable to explain how the light travelled through air, water and glass which took until around 1795 to resolve. Coincidentally, this date marks the birth of the poet John Keats, one of the renowned rhyme of ‘English Romantic’ poets. During his lifetime Keats would have been well aware of the acceleration in scientific discovery. Like other poets he began to rile against what he saw as the irrepressible march of science eradicating the mysteries behind physical and natural phenomena. There is an account of a conversation at a dinner with his fellow poet Wordsworth. Keats toasted ‘confusion to the memory of Newton’ Wordsworth then asked for an explanation of this and Keats replied ‘because he destroyed the poetry of the rainbow by reducing it to a prism’. Keats later expressed his sincerely-held but light-hearted irritation for Isaac Newton’s solution to the rainbow’s secrets in his poem:

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 gnomèd mine —
Unweave a rainbow, as it erewhile made
The tender-person’d Lamia melt into a shade.

Interestingly, a contemporary of Keats whose later writing made use of much scientific knowledge in his mysteries, piled in to add his criticism to the impact scientific progress was having. In a sonnet entitled: To Science, Edgar Allan Poe suggested that science is the enemy of not only the poet but society as a whole by removing the mysteries of the world. Poe was concerned by the ‘influx of these modern sciences’ and how these undermined spiritual beliefs and mysteries, supplanting them with cold, logical explanation. The first four lines read…

Science! true daughter of Old Time thou art!
Who alterest all things with thy peering eyes.
Why preyest thou thus upon the poet’s heart,
Vulture, whose wings are dull realities?

Like many such prodigies, Keats died young from tuberculosis, aged just 25 in 1821. Poe fared a little better just reached his 40th year when he passed in 1849, having suffered a brain seizure. So neither were around on 29th November 1859 when Charles Darwin published his: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. Just as well as it was to cause further consternation around the dinner tables of the day. That said, if they had suspended their anguish just long enough to at least read the concluding words of Darwin’s book they might have appreciated the words of a scientist who saw beauty in the wonders of nature:

“It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the conditions of life, and from use and disuse: a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.”

It is hard to find a better example of scientific writing describing with such elegance on such a profound subject, as in this instance by Darwin holding up a prism to the wonders of the natural world. Take time out to enjoy all of nature’s rainbows!

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