Newton and the colour of light

Why cutting a hole in your mum's shutters is good science.

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Sir Isaac Newton (1642-1727) exerted a profound influence on many aspects of science, notably on optics and dynamics, through his great mastery of precise experiments, but he was also a celebrated writer on religion, scientific method and the philosophy of science. He was born at the Christmas following the death of Galileo and would later declare his indebtedness to that Italian and to the Pole, Copernicus:

If I have seen a little farther than others it is because I have stood on the shoulders of giants

The Prism Experiment

Painting of Newton prism experiment

His investigations into optics commenced in 1666 at the end of an annus mirabilis when, at home in Woolsthorpe, Lincolnshire due to the bubonic plague which was raging in Cambridge, he investigated gravity, calculus and the laws of motion. He determined to ‘try therewith the celebrated Phaenomena of Colour’. It had been thought previously that colour was created by the mixing of light and darkness. Newton noted, however, that the blended print on the white page of a book appears grey, not coloured, when viewed from a distance. His experiments in bending light through prisms led, eventually, to the revolutionary discovery of the existence in white light of a mixture of distinct coloured rays, distinguishable when refracted in a prism. In his first experiment he projected the light via a round hole in his shutters.  

‘In a very dark Chamber, at a round hole, about one third Part of an Inch broad, made in the Shut of a Window, I placed a Glass Prism’. See Opticks, Prop. II, Theor. II Exper. 3. Newton began by projecting the light onto a wall before fixing the posture of the prism and projecting the light onto a white sheet of paper.

This produced a stretched image of the sun, which was mainly white, but featured a blue upper edge and red lower edge. In his second experiment he projected the light through a narrow slit in the shutters, thereby achieving the now familiar multi-coloured band. A painting in the BOA Museum shows Newton allowing light, via a prism, to reveal the spectrum on a piece of white card resting on a chair.

On his return to Cambridge Newton was unusually open about his discovery, demonstrating the prism experiment before his peers and showing that the colours could be recombined to form white light. He gave a detailed explanation of his discoveries in public lectures between 1669 and 1671 (published in 1728 as Lectiones Opticae) and in a paper to the Royal Society in 1672. In 1675, he presented another paper which described further experiments on the colour of thin films and plates, and which put forward a corpuscular theory of light which was surprisingly similar to the modern theory of light quanta. Newton’s Opticks, first published in 1704, went through many editions and was the most influential work on experimental science for almost all of the century. Amongst other things, it explained how raindrops refract sunlight to form rainbows. This was the first chromatic explanation of a phenomenon that had fascinated scientific writers, including Aristotle, Alhazen, Vitello and Antonio de Dominis, since the account of Noah’s Ark was first written down. He named the seven colours of the spectrum red, orange, yellow, green, blue, indigo and violet. These names have stuck, although the choice of seven should be seen as conveniently sacred, rather than a precise description of the visible spectrum. Close reading between the lines has shown that the Opticks is riddled with number symbolism. Newton also described how each colour of the spectrum merges gradually into its neighbour to give ‘hues’, though it was not until 1801 that Thomas Young, who had revived Huygens’ Wave theory of light, showed that the eye has three ‘cones’ or nerve endings to distinguish these hues. 

The Reflecting Telescope

Painting of Newton reflecting telescope

It was in 1668 that Newton made his first (in fact, the first ever) reflecting telescope, having abandoned the attempt to improve refracting telescopes. He constructed his own tools to manufacture some of the parts. The reflecting telescope used parabolic mirrors rather than lenses and thus avoided the problem of colour dispersion (‘chromatic aberration’) a phenomenon that could prove very distracting to viewers of a magnified image. This was a very practical solution and it has been suggested that Newton’s interests in practical alchemy and magical experimentation provided the necessary mindset to solve the problem.

The BOA Museum’s famous painting of Newton Investigating Light, shows one of his telescopes on the desk. It was much smaller than refracting models and swivelled on a wooden ball mount. His first model was just six inches long and one inch in diameter, yet had a x30 magnification power. Newton’s version did not, in fact, work particularly well, as the mirrors were prone to tarnish very quickly. 

The telescope did, however, allow further discoveries in astronomical observation. Since mirrors do not absorb light like lenses do, a reflected image of a distant planet (which is less bright to start with) can be viewed more clearly. This proved to be the case with Newton’s observation of the moons of Jupiter. Newton’s second model, some nine inches long and two inches in diameter, was examined by the Royal Society and shown to King Charles II by Barrow in 1671. Newton’s election as an FRS followed in 1672 and he delivered a paper to the Society later that year which met ‘both with a singular attention and an uncommon applause’.

Newtonian Telescope from 1820

Newtonian reflecting telescopes enjoyed a prolonged period of usefulness. The image to the right shows one that was featured in the Encyclopedia Londoniensis (1820).

An A3 poster featuring this image, amongst other telescopes may be purchased from the museum shop.

Newton's Flawed Genius

Newton’s researches in dynamics, begun during the Plague Years, when he gave a complete solution to the problem of colliding bodies and discovered the law of centrifugal force, were continued in 1679 and culminated in his book, Principia. One of his most original contributions to dynamics was his precise concept of force enshrined in his second law of motion. His contributions to science were matched by his great mathematical achievements, notably his discovery of the Binomial Theorem and the direct and inverse method of fluxions. Newton also exerted a profound influence through his views on scientific method, as found especially in the Queries to the Opticks and in the preface to the second edition of the Principia (1713).

His genius was marred by an intolerance of criticism and a jealousy for priority in his discoveries; indeed, his controversies with Robert Hooke, John Flamsteed, and G. W. Leibniz were marked by a bitterness remarkable even by the standards of his day. His agitated correspondence with Hooke and with the English Jesuits in Liege (who had failed to reproduce his prism experiment results) may have helped precipitate a nervous breakdown in 1678. It is significant that Newton felt unable to publish Opticks until the year following Hooke’s death. (He had in fact begun to summarise his optical ideas in writing in 1692 but the manuscript was destroyed by a fire in his college rooms).

Newton's Eyesight

Isaac Newton eyes

Whilst Newton suffered at various times from mental illness, his physical health remained robust; though he lived to his eighties he never needed to wear spectacles. He did, however, write to Henry Oldenburgh, Secretary of the Royal Society on 7 December 1675 explaining that ‘my own eyes are not very critical in distinguishing colours’. Newton risked his sight on several occasions carrying out pressure experiments on his own eyeballs to test the effect on colour vision of the curvature of the orbit. It has also been pointed out that, in his youth, Newton was near-sighted and therefore unable to make effective astronomic observations.

Despite the essential secretiveness of his character, pride ultimately caused Newton to disseminate his ideas on optics, dynamics, and scientific method. These ideas spread from England to the Netherlands, then to France and Germany, so that at the end of the century his influence was felt throughout Europe. His ideas were not universally accepted in later years, however. Goethe, for example, was highly critical in his Theory of Colours (1810). The man’s name came to be forever associated with ‘Newton’s Formula’ (for lenses), Newton’s Rings etc.

In 1703 Newton was chosen to be President of the Royal Society, a position he held until his death. He was knighted in 1705. On his death he was given a national funeral and buried in Westminster Abbey.

Wedgwood Newton plaque

Many commemorative items such as medals, statues and cameos were issued to mark the life of Newton. This is a Wedgwood & Bentley jasperware plaque. You'll find many more examples in our MusEYEum online catalogue.

Find out more about Newton's legacy in our Virtual colour vision gallery. His researches were also very influential in the future science of spectroscopy.