Light and Optics

An old engraving shows a man in a dark room passing a beam of light in front of a group of attentive people. — Isaac Newton (this is not certain) demonstrating an optical experiment to spectators in his laboratory: wood engraving by Martin after C. Laverie. Wellcome Collection, CC BY 4.0, source

In 1666, a young man who had withdrawn to the family farm because of the plague designed an experiment capable of proving him wrong.

It had long been known that sunlight passing through a prism brings out colours, and many scholars were working on the question.

In 1666, one of them, a young man of twenty-three who had withdrawn to the family farm because of the plague that had closed his university, carried out one of the clearest experiments on this site.¹

Its clarity lies not in what it showed, but in the way it was constructed: he designed it so that it could prove him wrong.

Light and its path

Light travels in a straight line as long as nothing deflects it. This explains shadows: an opaque object blocks the rays, and behind it its silhouette takes shape. A point source gives a shadow with sharp edges; a broad source gives one with blurred edges, because some points of the source still light the area that others no longer reach. This is why the shadow of a bird flying high is invisible: at that distance, the Sun is too broad.²

It moves at 300,000 kilometres per second in a vacuum — eight minutes to come from the Sun, a little over a second to come back from the Moon. Nothing goes faster. In water or glass, on the other hand, it slows down, and almost everything that follows stems from this slowing down.

On contact with matter, three phenomena can occur.

Reflection. Light bounces off. On a smooth surface such as a mirror or calm water, it bounces off in an orderly way and forms an image; on a rough surface such as a wall, it scatters in all directions. This is why we see the wall without seeing ourselves in it.
Refraction. On passing from one medium to another — from air to water, from air to glass — light changes speed, and therefore direction. This is what makes a spoon in a glass of water look broken in two, what makes spectacles, magnifying glasses and cameras work, and what spreads out the colours in a prism.
Absorption. Matter retains part of the light and turns it into heat. Black fabric absorbs almost all of it; this is why it heats up in the sun.³

A transparent object lets light through, an opaque object stops it, a translucent object lets it through while scattering it — like frosted bathroom glass, which lets in light without showing anything.

These three phenomena are enough to explain what this page recounts. The prism experiment is a matter of refraction; the colour of an apple, a matter of absorption and reflection.

The explanation of the time

The common explanation was that the glass coloured the light — through an impurity, through a property of the material, it hardly mattered. Something in the prism added colour to what was white on the way in.

There is nothing ridiculous about this explanation; it is in fact reasonable: one sees white go in, colours come out, and between the two there is an object. The object must therefore be doing something.

A first precaution, before any idea

It is worth noting that Isaac Newton’s first move is to improve the set-up.

He begins by darkening his room and making a small hole in the shutter to let in just the right amount of sunlight. Then he places his prism in the path of the beam, so as to project the colours onto the opposite wall — and not onto a screen pressed close behind the glass, as others had done.⁴

At that distance, the colours separate clearly.

The first advance comes not from a new idea, but from a change of set-up. This is more often the case than the usual accounts suggest.

One detail Newton does not hide: he had bought his prism at a fair. It was a child’s toy. He made it into an instrument.⁵

The crucial experiment

Then comes the essential part. Newton does not seek to confirm his intuition. He builds the experiment that the opposing explanation demands.

His reasoning can be followed step by step:

If it is the prism that colours the light, then a second prism should colour it further.

The set-up is simple. Newton places two boards, each pierced with a small hole: the first just behind the prism, the second a few metres away. Only a thin portion of the spectrum — a single colour — manages to pass through both holes. That light then goes through a second prism.⁶

He adds a refinement: he takes the first prism in his hand and turns it gently, so as to make all the colours of the spectrum pass one after another through the hole in the second board. He can thus observe, colour by colour, where each one lands on the wall.⁷

Two results are possible, and they have opposite meanings:

If the red comes out changed, then the prism does indeed colour the light, and Newton is wrong.
If it comes out unchanged, then the prism has merely separated what was already there.

Each colour comes out deflected, and unchanged. Deflected — that is refraction, and every prism does it. But none is broken down any further.

Newton then observes something else, which is the heart of his thesis: the colours are not deflected in the same way. Each has its own angle. The colour of a ray and the way it bends are linked; they are two sides of one and the same property, which belongs to the ray itself.⁸

The symmetrical check

He could have stopped there. Instead, he carries out the reverse experiment.

With a second prism, he recombines the spectrum into white light.⁹

If the prism added colour, this recombination would be impossible: you do not get white by adding still more.

Two opposite paths, a single conclusion: the colours are in the light, not in the glass.

A point of attribution

This experiment bears a Latin name, experimentum crucis — the crucial experiment, the one that settles the question.

Two clarifications are called for, since this site is wary of stories with a single hero.

The expression is not Newton’s. It appears in Robert Hooke’s writings in 1665, the previous year, and is rooted in the work of Francis Bacon, who was already speaking of instantia crucis in 1620.¹⁰

An earlier experiment deserves the name. In 1648, Blaise Pascal wants to test a hypothesis: if air has weight, then a column of mercury in a tube should fall as one climbs higher. He does not carry out the experiment himself — he is in Paris. It is his brother-in-law, Florin Périer, who carries the tube up to the summit of the Puy de Dôme on 19 September 1648. The structure is exactly the same — a prediction, a place to go and look, and a result that could have refuted it.¹¹

Newton did not invent the method. He provided its clearest example.

One last point, which matters for this site. This experiment did not win acceptance overnight. Criticised when it was published, in 1672, the theory it supported was accepted only gradually, over some thirty years — there were debates about the quality of the prisms, about how to repeat the test, about the conclusions to be drawn from it. That a demonstration is decisive does not mean that it convinces straight away.¹²

What colour is an apple?

If the colours are in the light, there remains the question of the colour of objects.

A red apple lit by daylight receives all the colours. Its skin absorbs most of them and sends back mainly the red. It is this reflected light that reaches the eye. Lit by a light lacking red — under certain street lamps, for example — the same apple looks dark, almost black.¹³

So “is” the apple red? It has a very real property: that of sending back one kind of light and absorbing another. This is not a matter of opinion. But the perceived colour also depends on the light that illuminates it. Colour is an encounter between the object, the light and the eye — not a paint applied to the object.¹⁴

As for the sky, one sometimes hears that it is blue because it reflects the sea. Not at all: the sky is just as blue above deserts. Air, too, does some sorting: it scatters the Sun’s blue light in all directions far more than the red. Wherever one looks in the sky, it is this scattered blue that reaches the eye. At sunset, the Sun’s light passes through so much air that the blue has been scattered elsewhere: only the red remains.¹⁵

Note 1

Isaac Newton was born here on Christmas Day 1642, in the tiny hamlet of Woolsthorpe in Lincolnshire. His father was an illiterate but prosperous farmer who lived with his wife Hannah, and on their 100-acre farm they raised sheep and cattle, and grew hay, oats and corn. […] By the summer of 1665 the plague had reached Cambridge, and the University closed its doors and sent everyone away. Newton returned to Lincolnshire and continued his investigations into mathematics, optics and gravity (as depicted here).

Flood, Raymond and Wilson, Robin, “Isaac Newton: life, labours and legacy”, lecture, Gresham College, 28 September 2020, transcript

Although by the calendar in use at the time of his birth he was born on Christmas Day 1642 […] his scientific genius had still not emerged, but it did so suddenly when the plague closed the University in the summer of 1665 and he had to return to Lincolnshire.

O’Connor, John J. and Robertson, Edmund F., “Isaac Newton”, MacTutor History of Mathematics, University of St Andrews, January 2000

in the beginning of the Year 1666 […] I procured me a Triangular glass-Prisme, to try therewith the celebrated Phænomena of Colours.

Newton, Isaac, “A Letter of Mr. Isaac Newton… Containing His New Theory about Light and Colors”, Philosophical Transactions of the Royal Society, vol. 6, no. 80, 1672, p. 3075-3087