THE COLOURS OF THE RAINBOW
Isaac Newton is widely regarded as the greatest scientific thinker that ever lived, and the emphasis in this appreciation is usually on his skills as a theorist, propounding the laws of motion and, most famously of all, the law of gravity. But like his contemporaries, Newton was also a ‘hands-on’ scientist – a practical man who did his own experiments, often using equipment he had designed and built himself. This approach transformed science in the 1660s, largely because of the influence of the Royal Society, founded in the early years of that decade (it received its Royal Charter in 1663). The motto of the Society was (and is) Nullius in Verba, which can be loosely translated as ‘take nobody’s word for it’. From the outset, they did not simply accept hearsay reports of scientific discoveries, but carried out experiments and demonstrations themselves to test such claims. (In the early days, Hooke was the man who did the experiments.) Newton came to the attention of the Society in 1671 because of his practical skills – he had designed and built a new kind of telescope, useful for astronomical investigations, which focused light using a curved mirror rather than a lens. The telescope was shown to the Royal Society by Isaac Barrow, a Cambridge mathematician who had been one of the first people to recognize Newton’s ability. Newton himself was by then Lucasian Professor of Mathematics in Cambridge, but lived a quiet life and largely kept his many discoveries to himself.
© David Parker/Science Photo Library
Isaac Newton portrayed in front of his own drawing (colour added) showing the splitting of white light from the sun into the spectrum. The illustration also shows that a second prism refracts the original colours, in this case red, without further change once split, or dispersed, the second prism refracts the original colours, in this case red, without further change.
The Society was sufficiently impressed to elect Newton as a Fellow on 11 January 1672, and to ask him what else he had been working on. His reply took the form of a long letter (what we would now call a scientific paper) in which he explained his ideas about light and the experiments on which those ideas were based. Newton’s key insight was that ‘pure’ white light is actually a mixture of all the colours of the rainbow. To the ancients, white light represented a pure entity, in the same way that spheres were thought to be perfect. To suggest that white light was a mixture of colours would have seemed to them as ludicrous as the idea that planets did not move in perfect circles in their orbits.
Of course, it was well known that when sunlight passed through triangular prisms or other pieces of glass it produced rainbow patterns of colour. But before Newton people assumed that the white light passing through the glass had become adulterated as it picked up imperfections from the glass, and this had changed its nature. Newton’s genius was to devise a simple experiment which proved that this was wrong.
In the first stage of the experiment, he worked in a darkened room with heavy curtains to shut out the sunlight. A small hole in the curtain let through a single beam of light, which fell upon a triangular prism. After passing through the prism, the light shone upon a white wall on the other side of the room, where it was spread out into a rainbow pattern of colours. It was Newton who identified seven different colours in this pattern – red, orange, yellow, green, blue, indigo, and violet.
This could still be explained as a change brought about by the passage of the light through the glass. But in the second part of the experiment Newton put a second prism, reversed compared with the first prism, between the first prism and the white wall. The first prism had spread white light out into seven colours; the second prism did not make the colours even brighter (more ‘impure’) but rather combined the seven colours back into a single spot of white light. He had turned a ‘rainbow’ back into white light. The explanation was that white light is really a mixture of all the colours of the rainbow, and that light is bent as it passes through the prisms (or, indeed, inside raindrops). Some colours are bent more than others, so they get spread out or squeezed back together depending on how the prism is oriented. It is as easy to bend them back into a single beam as it is to bend the different colours out of a single beam. This was the first step towards an understanding of spectroscopy (see here), which would one day make it possible to determine the composition of the stars, using reflecting telescopes, some of them developed from Newton’s design.
This was just one of Newton’s insights into the nature of light, which were eventually gathered in a great book, Opticks, published in 1704, the year after he had become President of the Royal Society. In that book, he summed up his own understanding of the scientific method: ‘Analysis consists in making Experiments and Observations, and in drawing general Conclusions from them by Induction, and admitting of no Objections against the Conclusions but such as are taken from Experiment, or other certain Truths.’
© Universal History Archive/UIG/Science Photo Library
Replica of Newton’s reflecting telescope.
| No. 12 | THE SPEED OF LIGHT IS FINITE |
Newton’s remark about experiments ‘and Observations’ is important. Sometimes Nature does the ‘experiment’ for us, and the role of the scientist is ‘only’ to observe what is going on and work out why it has happened. But it often takes a very clever person to work this out. Ole Rømer’s discovery of the finite speed of light is a case in point.
In the seventeenth century, there was a great deal of interest in using studies of the eclipses of the moons of Jupiter (discovered by Galileo) as ‘clocks’ to determine longitude. These eclipses occur at regular intervals, as the moons orbit Jupiter in the same way that the Earth orbits the Sun. The moment when a particular moon disappeared behind Jupiter could be observed from different places on Earth, and the time at which it occurred could be compared with the local time measured from noon. This told the observers how far east or west they were from some chosen reference point. This technique was pioneered by the Italian Giovanni Cassini, who moved to the Paris Observatory in 1671. He sent the Frenchman Jean Picard to Denmark to use observations of Jupiter’s moons to establish the exact longitude of Tycho Brahe’s old observatory, so that Tycho’s records could be tied in to observations from Paris. Picard was helped by a young assistant, the Dane, Ole Rømer, who then went to Paris to work with Cassini (he was also for a time the tutor of the French Crown Prince, the Dauphin).
Over the next few years, Rømer continued to monitor the eclipses of Jupiter’s moons, and noticed that these did not always occur exactly when they were expected. He made a particular study of the moon Io, and noticed that the time between one eclipse and the next got shorter when the Earth was moving towards its closest to Jupiter (which happens when it is on the same side of the Sun as Jupiter), and longer when it was moving further away. Cassini himself thought for a time that this might be because light travels at a finite speed. When the Earth is moving towards Jupiter, the time between successive eclipses is shorter because in the time between eclipses the Earth has moved closer to Jupiter, so light from the second eclipse does not have so far to travel and gets here quicker. Similarly, when the Earth is moving away from Jupiter, the light from the second eclipse has further to travel, because the Earth has moved on in its orbit, and takes longer to reach us.
Curiously, Cassini abandoned the idea. But Rømer took it up, and made detailed observations and calculations to develop it further. In August 1676, Cassini, then still enamoured of the idea, announced to the French Academy of Sciences that the official tables of the eclipses of Io, used in calculating longitude, would have to be revised ‘due to light taking some time to reach us from the satellite; light seems to take about ten to eleven minutes [to cross] a distance equal to the half-diameter of the terrestrial orbit.’ Cassini also predicted