Newton self-isolates

Newton’s prism experiment retold.

As a way of putting enforced seclusion to good use, it’s hard to beat Newton’s optics.

You all know the story in outline. In 1665 the bubonic plague that devastated London reached Cambridge, where Newton was a freshly minted B.A. (Cantab.) He fled to his uncle’s farm in Lincolnshire. This is now called Woolsthorpe Manor, though it’s more the farmhouse of a prosperous yeoman. He took with him a pair of prisms, with which he destroyed the prevailing theory of colour with a devastating experiment. We all know that Newton discovered, or rediscovered, the colour spectrum using a glass prism placed in a beam of light. But the real breakthrough came from the second prism.

Brief flashback. This prevailing theory was a common-sense one. White sunlight passes through a stained-glass window. It becomes blue or red or yellow. It’s the medium, the stained glass, that gives the colour, right?  As Shelley wrote, 150 years later:

Life, like a dome of many-colored glass,
Stains the white radiance of eternity.

Wrong.

The usual story is that Newton hid out in a barn. His own sketch of the experiment disproves this.

Barns don’t have small, square, high windows. Bedrooms do – like those on the first floor of the house. Glazed windows were gradually adopted in England in the course of the 17th century. As like as not, the bedrooms in the Newton farmhouse still only had stout wooden shutters to keep out the cold Lincolnshire winds: shutters with cracks in them.

The sketch clearly shows what Newton did. He placed prism 1 in a beam of sunlight passing through a crack or hole in the shutter, producing the familiar spectrum. Where did these colours spring from? Perhaps it was the medium again, the prism glass, as Descartes had proposed. Newton constructed a screen with a ladder of more holes to allow him to isolate the different colours. He placed prism 2 in the red, blue, .. light – and it stayed put. Schematically:

It doesn’t make sense that prism 1 would create colours and identical prism 2 do nothing to them. (To be quite sure you would need to replicate with prisms made from different sources of glass, but that was quickly done.) So the colours were in the light to begin with.

The illusion is white light, really a bundle of different colours. More disturbingly to our intuition, a perceived colour is a negative property. The stained glass absorbs all the other colours than the blue we see. Leaves are green because that wavelength is not absorbed by chlorophyll, which is tuned to blue and red.

It took Newton five years to write this all up into a full theory of optics. It was his 1672 paper on this that made him deservedly famous. Gravity came later (1687), though he started on that in the farmhouse too.

Go on. Crack string theory.

Author: James Wimberley

James Wimberley (b. 1946, an Englishman raised in the Channel Islands. three adult children) is a former career international bureaucrat with the Council of Europe in Strasbourg. His main achievements there were the Lisbon Convention on recognition of qualifications and the Kosovo law on school education. He retired in 2006 to a little white house in Andalucia, His first wife Patricia Morris died in 2009 after a long illness. He remarried in 2011. to the former Brazilian TV actress Lu Mendonça. The cat overlords are now three. I suppose I've been invited to join real scholars on the list because my skills, acquired in a decade of technical assistance work in eastern Europe, include being able to ask faux-naïf questions like the exotic Persians and Chinese of eighteenth-century philosophical fiction. So I'm quite comfortable in the role of country-cousin blogger with a European perspective. The other specialised skill I learnt was making toasts with a moral in the course of drunken Caucasian banquets. I'm open to expenses-paid offers to retell Noah the great Armenian and Columbus, the orange, and university reform in Georgia. James Wimberley's occasional publications on the web

One thought on “Newton self-isolates”

  1. As the saying goes, it’s good to be the king. I am no Newton. One way in which I am not Newton is that if it were me, I’d wonder if maybe I had split them into primary colors somehow. Not that I really understand those in a physics way. (I was better at chemistry.)

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