Different light: Newton and the spectrum

This is the next sample from They Saw the Difference.

When I was three, my bedroom had the leadlight windows that were popular as a bit of middle-class poshness in the 1930s, when our house was built. These windows were made of small pieces of glass, held together between H-shaped strips of lead. Each window had one special piece of extra clear glass with bevelled edges, and these gave me one of the wonders of my youth.

The windows faced west, and at certain times of the year, when the low afternoon sun shone between the houses across the road, it hit these angled pieces of glass, and a small coloured patch appeared on my wall—my own private rainbow, captive in my room. 

Any triangular piece of glass will bend light, and the light is split because each of the colours is refracted by a different amount, because each colour has a different wavelength. If we want to see the effect well, it is best to use a narrow beam of white light, and pass it through a prism in a darkened room. Newton saw it, and wondered why the light was different.

A prism is just a solid figure that is essentially triangular in shape and made of a transparent material. Prisms are commonly used in physics to deviate or disperse a ray in optical instruments or laboratory experiments, or to deliver total internal reflection. Here is how Newton set up his investigation:

 

In this diagram, white light comes
from the right and is dispersed.

One thing is certain, this three-year-old was too late to make any original discoveries, because Newton completed his systematic study of the spectrum, long before I was born. In 1666 he saw the composite nature of white light while carrying trying to minimise chromatic dispersion in lenses, an annoying effect that had been known for about fifty years, when telescopes and microscopes gave images with coloured fringes. 

In 1672, Newton told the world how he had studied the ‘celebrated phenomenon of colours’. At the time, most people assumed that colour was a mix of light and dark, that the prism somehow added the colour. and Robert Hooke was one of the strongest supporters of this view.

With two neat experiments, Newton demolished Hooke’s ideas, and began one of the great feuds of science. (In case you don’t know it yet, science is driven by spotting differences, but there’s always room for personal differences, and many of the wildest brawls revolved around Newton. By comparison, Dart’s clashes with the Piltdown gang {see the last blog} were nothing!) 

In one experiment, Newton used a second prism to pull the colours back together again, and showed that the result was white light. Then he used a second prism and a slit to show that when a selected band of coloured light passed into the second prism, it passed on unchanged. Hooke’s theory was in tatters, and Newton had an enemy for life. Newton wrote no more on optics until after Hooke died in 1703. 

 In fairness to a rather peculiar man, Newton simply could not help bursting out with the truth, even if it got up people’s noses! Novelist Aldous Huxley assessed Newton this way: 

If we evolved a race of Isaac Newtons, that would not be progress. For the price Newton had to pay for being a supreme intellect was that he was incapable of friendship, love, fatherhood, and many other desirable things. As a man he was a failure; as a monster he was superb.

How Newton’s experiment
is often wrongly shown.

Newton’s experiment is often shown like this, upside down, with a triangular prism sitting on its base, the light coming from below, bending down as it passes through the apex and being directed further down on the other side. Newton used sunlight shining down (not up!) through gap in a window, and being bent through the apex of an upside-down prism to shine upwards onto a wall, 22 feet (7 metres) away. 

We know the sitting-on-its-base-prism picture is wrong, because of the angle of the incoming beam, and besides, the band at the top is ultraviolet (above violet), while that at the bottom is infrared, meaning below red. The prism had to be point-down. The room must have been darkened, with just one beam of light entering the room and throwing a pale spectrum onto the wall, red at the bottom, violet at the top. 

Newton called the colours ‘spectrum’, a Latin word meaning spectre or apparition, and he is said to have been the first to see a prism, but I wonder how many others had found their own private rainbows before him, given that he spoke of the ‘celebrated phenomenon of colours’. 

Those celebrated colours were just those of the rainbow, but how many were there? Almost everybody had their own version. In fact the spectrum contains an infinite number of colours, and the number we ‘see’ is subjective. Newton followed a Greek astronomer named Ptolemy who had said there were seven colours, and gave them the names we use today: red, orange, yellow, green, blue, indigo and violet. 

 He said he could not separate any further colours from any narrow band of light selected from the spectrum, but his set-up would never have given monochromatic light in any selected band, because the rays from the Sun are not completely parallel. He should have seen further separation, but perhaps he intended his ‘result’ to be taken only as an idealised case. Or maybe he fudged his experiment: he stated correctly that different colours are refracted (bent) through different angles, both in prisms and in lenses, and that was the important part. 

What we now call visible light is just a small part of a much larger electromagnetic spectrum, but as we will see in chapter 5, this took time to find, and it is important to note that this is just the range that we humans see. There are many insects, bees for example, which are different because they can see ultraviolet light. 

So how do you see something that cannot be seen? In 1799, an astronomer named Sir William Herschel was measuring the temperatures associated with different colours. He had been using filters to view the Sun, but he saw how some filters that he used when examining sunspots let more heat through.

This led him to wonder if different colours had different amounts of heat, and he used thermometers to measure the strength of heating along a normal spectrum. He did this because his observations made him speculate that: 

 …the prismatic rays might have the power of heating bodies very unequally distributed among them…If certain colours should be more apt to occasion heat, others might, on the contrary, be more fit for vision by possessing superior illuminating power. 

Herschel found a higher temperature near the red end, and testing just beyond the visible range, found an even higher temperature, so he named this radiation ‘calorific rays’. He showed that these invisible rays behaved like visible light, being reflected, refracted and transmitted, the same tests Heinrich Hertz later applied to his radio waves. 

Herschel delivered a number of papers on the subject to the Royal Society in 1800, describing several hundred experiments on what he called “invisible light”, and since then, infrared astronomy has increased in importance as more sensitive instruments to detect infrared radiation have been developed. We will look more at infrared and infrared astronomy in the book. 

Johann Ritter tested the ultraviolet end of the spectrum, using silver chloride to detect radiation beyond violet. Photographers would later use the way light makes silver chloride go black, but Ritter found that there was an invisible band of radiation, even better than white light at blackening the silver chloride, which we now call UV. 

To sum up, the visible part of the electromagnetic spectrum, ranging in wavelength from approximately 3.9×10-7m (violet) to 7.8×10-7m (red) (corresponding frequencies 7.7×1014 Hz and 3.8×1014 Hz, respectively). 

As Aldous Huxley reminded us, Newton was unpleasant. He mistreated Stephen Gray, he quarrelled with Robert Hooke over the inverse square law and his theory of colour, with Gottfried Leibniz over the invention of calculus, and with Christiaan Huygens over his theory of light. 

Even so, any one of Newton’s achievements would have been enough to ensure his fame, even without the apple. But did the apple really fall? We will never know now, but the tale was made popular by Voltaire, and Newton’s biographer and friend, William Stukeley, claimed Newton told him the story, so maybe there was a day when the apple fell, and made Newton wonder why it should be so. That is Newton’s greatest gift to us, that he asked why as often as he did, even inspiring poets like Paul Valéry and Alexander Pope, who wrote: 

Nature and Nature’s laws lay hid in night:

God said ‘Let Newton be.’ and all was light. 

Mind you, Sir John Collings Squire would later add:

It did not last: the Devil shouting ‘Ho, 

Let Einstein be.’ restored the status quo. 

Newton told Hooke that if he had seen further than others, it was because he had stood on the shoulders of giants. This may have been a snide dig at Hooke, but a similar remark had been made by many others, centuries earlier. Robert Merton has even written a whole delightful book (On the Shoulders of Giants), on the subject, tracing the earlier history of the aphorism.

It was once cited with this brilliant typo: 

Merton, Robert K., On the Shoulders of Grants: A Shandean Postscript, Harcourt Brace, New York, 1965. 

—Max Charlesworth, Lyndsay Farrall, Terry Stokes, David Turnbull, Life Among the Scientists, Oxford University Press, 1989, bibliography, 295.

Next, we'll look at the spectroscope and what it can do.