17﷓ ISAAC NEWTON

From "Great Scientific Experiments" Rom Harre

17‑ ISAAC NEWTON

The Nature of Colours

Isaac Newton was born at Woolsthorpe in Lincolnshire on Christmas Day, 1642. His father had died before he was born, and his mother married again when he was only two. As a child he demonstrated his manual dexterity as he `busied himself making models of wood in many kinds'. Most of his childhood was spent with his grandmother. He went away to school at Grantham, and then on to Cambridge in 1661, but not before he had tried his hand at farming without a great deal of enthusiasm.

Newton was very successful at Cambridge. He was elected to a minor Fellowship at Trinity College in 1667 and became a major Fellow in 1668. In 1669, at the age of twenty‑six, he was elected to the Lucasian Chair of mathematics.

The Great Plague had closed the university in 1665, and Newton retired to his mother's farm at Woolsthorpe. His great productive period had begun in about 1664. The falling apple that sparked off his theory of universal gravitation is said to have come from one of the trees in the Woolsthorpe orchard. Between 1665 and 1667 he developed the method of fluxions (the calculus, as we now call it), carried out most of his experimental work on the nature and properties of light, and laid the foundations of the universal mechanics in which he synthesized the terrestrial science of Galileo with the planetary theory of Kepler. But he took many years to prepare these discoveries and inventions for publication. Newton was very sensitive to criticism, and the equivocal reception of his first communication to the Royal Society, on the nature of light, made him wary of publishing mere fragments of research. St) we find him holding on to his discoveries until they could be worked up into massive treatises. The Principia, the great work in which he set out his mechanics and cosmology, did nw appear until 1687. The Opticks, most of the experimental work for which had been done around 1666, was finally published only in 1704.

In 1689 Newton took his seat in the House of Commons as a Member for Cambridge. This event marked a considerable change in his interests, and some historians have suggested, in his character. He virtually abandoned scientific research from about this time, and enjoyed the life of a senior administrator and public figure. He became Master of Royal Mint and is said to have run it with exemplary efficiency. Throughout his life he had taken an intense interest in theological matters. Even in old age he was still trying to solve chronological problems in the dating of events recorded in the Old Testament. He died in 1727, having acquired a reputation in his own life‑time that no other scientist was ever quite to have again.

Early work on light and colour

Is colour a quality of light produced in a body, or is it a quality separated out of light by a body? This seems a question of some profundity and its solution likely to be of great technical difficulty. The problem had a long history. Theodoric of Freibourg, whose masterly solution of the difficulties of understanding the rainbow we have studied above, was typical of medieval thinkers in generalizing a vaguely Aristotelian explanation. He thought that light acquired its colour from the medium through which it passed. His explanation is based upon the idea of pairs of contrary principles. A medium can be more or less translucent. Near the surface a medium is more bounded than it is in its depths. A mirror is perfectly bounded, and reflects all light, having no effect on colour. A transparent solid is unbounded, allowing light to penetrate deep into its interior. White light is passed by a medium having a perfect balance of the four contraries. When a medium is relatively bounded, that is near its surface, light is qualitatively changed so as to appear red. But when the medium is relatively opaque in its interior, the light is so changed as to appear blue.

Fig. 30. The separation of rays of different 'refrangibility'. Newton, Opticks (1721 edn1, book 1, part 1, table iv, fig 18. S is the source of white light. In prism ABC the rays of different refrangibility are separated. The screens DE and de serve to separate progressively purer colours.

This explanation could hardly be counted very satisfactory since the contraries seemed rather more mysterious than the production of colours they were called upon to explain. A closer study of the way light was affected by transparent objects showed that the colours had something to do with the way light was refracted when passing from one medium such as glass to another, such as air. Descartes was the first to separate light of pure colour using this effect. In Les Meteores of 1637 he describes an experiment which he had performed in the course of studying the rainbow. The experimental arrangement is shown in Figure 20 ("Theodoric"). `When I covered one of these surfaces with a screen,' says Descartes, `in which there was a small opening DE, I observed that the rays which pass through this opening and are received on a white cloth or sheet of paper show all the colours of the rainbow; and that the red always appears at F and the blue or violet at H.'

What relation did these coloured rays have to the light fron the sun which had fallen on the prism? It was to the answer t~ this question that Newton's experiment was addressed.

Newton's systematic research programme

Newton's series of more and more successful versions of the basic experiment to be described here was not original in conception, but it was to develop into a fairly exact execution (For an account of the forerunners of Newton in the study of colour and refraction see J. A. Lohne, Notes and Records of the Royal Society of London, 20, 1965, pp. 125‑39.) In his letter to the Royal Society of 1672, Newton tells of the puzzlement he felt, when in an experiment of 1666, he noticed that the shat of the spectrum image cast on a screen by passing light from round hole through a prism, was oblong, `with straight sides' he says. Why should this be so? According to Lohne (see Further Reading), Newton must have tidied up his description of this image somewhat, since the greater intensity of the yellow component in the sun's light would have made the image rather broader at that point in the spectrum.

In preparing a definitive account of the experiment for the Opticks, Newton describes how he took pains to refine and sharpen the image. `By using a larger or smaller hole in the window‑shut [he] made the circular images larger or smaller at pleasure. The amount of light could be increased by using a narrow oblong hole rather than a circular one, keeping the ends of the spectrum image sharp.' Newton seems to have ignored or overlooked diffraction effects of the use of a small hole as image, though these had been noticed by his contemporaries.

The basic experiment, refined by the use of a lens to focus the image of the hole, was quite simple: The spectrum is thrown on a piece of black paper in which there is a small hole. When the hole coincides with the red part of the spectrum a beam of red light is obtained, which can be refracted through a second prism. Similarly when the hole coincides with the blue part of the spectrum a blue beam is separated out. It is the effect of the second prism that is the key. There are two results to be noticed. The resulting image, whatever its colour, is quite circular, `which shows that the light is refracted without any dilatation of rays', since the shape of the hole is perfectly reproduced in the image. But when a blue ray passes through the second prism it is more refracted than a red ray. So the separation of the colours is a secondary effect. The underlying process is the separation of `rays of different refrangibility'. In a letter to Lucas of 5 March 1677/8, Newton was at pains to emphasize the true result of the experiment. `. . . you think I brought it to prove that rays of different colours are differently refrangible: whereas I bring it to prove (without respect to colour) that light consist of rays differently refrangible. What the colours of the rays differently refrangible are . . . belongs to after enquiry . . .' (quoted by Lohne).

What is probably the last of Newton's many versions of the experiment is illustrated in the engraving to be found in the Paris edition of the Opticks. It was drawn from a sketch supplied by Newton himself (cf. Lohne, 1968).

Fig.31. The effect of using light sources of different shapes.

So far Newton had achieved no more than a more exact repetition of the cruder experiments of his predecessors. Even the testing of monochromatic light by passing it through a second prism had been anticipated, albeit crudely, by J. M. Marci of Kronland. Marci was a prominent physician in Prague. Though isolated from contacts with Western scientists by the Catholic reaction in Bohemia in the early seventeenth century, he did important work in astronomy, optics and medicine. But though he succeeded in decomposing white light into coloured beams, it was to be left to Newton successfully to reconstitute the original beam.

But to demonstrate that the phenomenon of colours in refracted light is caused by the different refrangibility of rays already present in the white beam, and not by some modification produced in the light by the glass of the optical apparatus, something more is needed. Newton's original recombination experiment reported in the Letter of 1672 involved the use of a lens to bring about the confluence of the rays. The reactions of many of Newton's contemporaries to the experiment were tepid. Hooke objected that the experiment does not show that the light, prior to refraction, should be thought of as a collection of these different rays. They could have been produced in the process of refraction. However, in the Opticks Newton added another and very ingenious recombination experiment to refute this kind of objection.

Fig.32. Decomposition, recomposition and decomposition of white light to the spectrum. Newton, Opticks (1721 edn), book I, part II, table iv, fig.16. Rays refracted by prism ABC are recombined optically by lens MN, and are reseparated by prism KIH.

By using a long, flat prism, Newton made the angle which separates the beams of coloured light very small. By altering the angle of a screen arranged as in Figure 32, colours can be produced from what looks like white light. When the screen is at position B, there is enough diffusion of light caused by dust particles in the air for the narrowly separated coloured beams to be mixed again. By altering the angle of the screen to position C the coloured beams are made to strike the screen at sufficiently separated places for a spectrum to be seen. The distance WZ, separating the points of contact of the red and blue beams with the screen in position C, is much greater than the distance XY separating the images from the red and blue beams when the screen is in position B. The only feature of the arrangement which varies is the angle of the screen. The separation of images is being brought about by manipulating something quite independent of the prism which is producing the original, narrowly differentiated beams. Altering the angle of the screen allows the differently coloured rays to be identified without the diffusion of light from one beam to another which occurs when the images are very close together.

Fig.33. Recombining colours without a lens.

To clinch the matter Newton undertook a much greater variety of optical manipulations than Marci had attempted. Newton showed that once the colours had been properly separated they were unaffected by any of his manipulations. Refraction and repeated refraction did not change the colour.

In a typical refraction experiment Newton illuminated an object with monochromatic light, and then looked at it through a prism. If the passage of light from the object to the eye through the prism had had any effect on the light then he should have seen some difference in the colour of the thing when so observed. `But those illuminated with homogeneous light appeared neither less distinct, nor otherwise coloured, than when viewed with the naked eye.' Newton remarked that since the differences between the rays might really be continuous, light could not be perfectly homogeneous, no matter how sharply focused. But the spread of colours in each apparently homogeneous ray is so small that `change was not sensible, and therefore in experiments where sense is the judge, the change ought not to be considered at all'. Truly homogeneous light cannot be produced by refraction. Modern lasers which do produce perfectly coherent light depend upon a different physical principle.

The final step was to examine a wide variety of substances, `paper, ashes, red lead, gold, silver, copper, grass, blue flowers, violets, bubbles of water tinged with various colours, peacock's feathers and such like . . .' Under red light, they all appeared red. Under blue light they all looked blue, under green light, green and so on. Reflection, like refraction, has no effect on the colour of relatively homogeneous light.

The study of colour after Newton

But why are these results so readily and unambiguously obtained? Newton and Descartes before him had supposed that in some way or another the motion of particles was involved in the transmission of light. Newton considered the speed of the particles to be the cause of our experiences o~ colour, while Descartes thought it had to do with their rate o1 rotation. Eventually the problem was solved, at least relative to the known phenomena, by Euler. About the year 1746 he gave precise mathematical form to another rival theory that had been proposed, notably by the Dutch physicist, Huyghens. Euler showed that Newton's experimental results and many other phenomena could be elegantly explained by assuming that light was propagated as a wave in an all‑pervasive medium, the luminiferous ether. Light was not to be thought of as a stream of particles, but as vibration in an elastic solid. Colours corresponded to waves of different wavelength. This explained why different colours were differentially refracted when they passed from one medium to another. The colours were not produced in the medium, as medieval physicists had thought, but at the boundary between media. Elegant though Eider's solutions were, they too have to be modified under the pressure of still more recondite discoveries about electromagnetic radiation of which light is only one rather special kind.

In most of the experiments preceding Newton's study of colour, the subject under investigation lay ready to hand in the common experience of mankind. Falling bodies, compressed gases, the rainbow and its accompanying drops of rain, even the developing chick, are all within the range of our senses. In the conclusion Gilbert drew from Norman's experiment a more subtle kind of being is proposed, something no human observer could ever experience. The orbis virtutis is the unobserved or `occult' cause of observable magnetic effects. For all their apparent simplicity Newton's experiments on colour also go beyond experience, though not so deeply as those of Norman and Gilbert. Newton's refractions and screenings show that white light (which can never be perceived by us as other than white) is `really' a mixture of coloured rays, which can be perceived as they are, only when separated from all others by some accidental or human manipulation.