Physics 1704

Opticks: or, A Treatise of the Reflections, Refractions, Inflections and Colours of Light

Isaac Newton

White light is a mixture of colours; a prism only sorts them, it never makes them.

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In depth · the introduction

Newton took the most ordinary thing in the world — a beam of sunlight — and proved it was secretly a whole crowd of colours travelling together.

The big idea

Hold a glass prism in a sunbeam and a band of rainbow colours appears on the wall, always in the same order. The easy assumption is that the glass somehow stains the light. Newton showed the opposite: the colours were inside the white light all along, and the prism merely spreads them apart, because each colour is bent by its own fixed amount — violet most, red least.

His clinching move was to put the rainbow back together. Catch the spread colours with a lens, or a second prism turned the other way, and they merge into white again. White light is not pure or simple; it is every colour mixed. A prism doesn't make colour — it sorts it.

How it came about

A young Newton, sent home from Cambridge during the plague years of the mid-1660s, bought a glass prism — by his own account at a country fair — and darkened his room to play with sunbeams. The puzzle that caught him was the shape of the patch on the wall: a round hole should have thrown a round spot, but the prism stretched it into a long coloured streak. That oddity led him to the idea that white light is a mixture.

He announced it in 1672, in his first scientific paper, and was met with a storm of objection — most fiercely from Robert Hooke. Newton hated the wrangling so much that he all but withdrew from optics for three decades. He finally published the Opticks in 1704, in plain English and as a sequence of experiments anyone could repeat — and, pointedly, only the year after Hooke had died.

Why it mattered

Once you know white light is a mixture of fixed colours, you can take any light apart and read it. That is spectroscopy — and it became one of science's most powerful tools, letting us identify the chemical elements in a flame, and then in the Sun and distant stars, simply from the colours they emit. The same discovery explained the rainbow, gave colour a place in physics, and pushed Newton to invent the reflecting telescope to dodge the colour-blurring of ordinary lenses.

A way to picture it

Think of a chord played on a piano. It reaches your ear as a single sound, but it is really several notes struck at once — and a trained ear can pick them out one by one. White light is that chord; the prism is the ear that separates it into its notes. Play the notes together again and you hear the chord; bring the colours back together and you get white.

Where it sits

Descartes had already worked out the geometry of the rainbow, but not why it was coloured; Newton supplied the why. His great rival on the nature of light was Christiaan Huygens, whose 1690 wave theory (also in this Library) competed with Newton's picture of light as tiny particles. Newton's authority kept the particle view on top for a hundred years — until Thomas Young's interference fringes (1804, in this Library too) revived the wave, Fresnel gave it mathematics, and Maxwell revealed light to be an electromagnetic wave. The final twist came with quantum physics, which found that light is, after all, both wave and particle — though the modern photon is nothing like Newton's corpuscle.

The original document

Original source text

Definitions & Axioms

Isaac Newton · Opticks, Book I, Part I · first edition London 1704 (text of the 1730 edition; spelling as printed)

The Opticks opens not with a hypothesis but with definitions and axioms, in the manner of a geometry. Newton first fixes what he means by a ray, and then by the single property on which the whole book turns — refrangibility, a ray's disposition to be bent in refraction.

By the Rays of Light I understand its least Parts, and those as well Successive in the same Lines, as Contemporary in several Lines.

Refrangibility of the Rays of Light, is their Disposition to be refracted or turned out of their Way in passing out of one transparent Body or Medium into another. And a greater or less Refrangibility of Rays, is their Disposition to be turned more or less out of their Way in like Incidences on the same Medium.

The Light whose Rays are all alike Refrangible, I call Simple, Homogeneal and Similar; and that whose Rays are some more Refrangible than others, I call Compound, Heterogeneal and Dissimilar.

Axiom V — the sine law, with Newton's ratios

The Sine of Incidence is either accurately or very nearly in a given Ratio to the Sine of Refraction.

Thus if the Refraction be made out of Air into Water, the Sine of Incidence of the red Light is to the Sine of its Refraction as 4 to 3. If out of Air into Glass, the Sines are as 17 to 11. In Light of other Colours the Sines have other Proportions: but the difference is so little that it need seldom be considered.

Book One — that white light is compounded

Letting a sunbeam in through a small round hole and a prism, Newton expected — on the old view that refraction merely bends light — a round patch of colour. Instead he found an oblong spectrum about five times longer than it was broad, red at one end and violet at the other. One bending angle cannot stretch a round hole into a streak, unless the beam was already a mixture of rays bent by different amounts.

The Light of the Sun consists of Rays differently Refrangible.

To rule out that the prism was somehow tainting the light, Newton isolated a single colour through a slit and sent it through a second prism: it bent again by its own fixed amount and emerged the very same colour, never dividing further. Refraction sorts the rays; it does not manufacture the colour. Pure, single-refrangibility light he called homogeneal; sunlight, compound.

[ … ]

The decisive proof ran the experiment backwards. Gathering the spread spectrum with a lens, or with a second inverted prism, Newton recombined the colours — and out came white light again. White is therefore not a simple colour but a due mixture of them all.

Whiteness and all grey Colours between white and black, may be compounded of Colours, and the whiteness of the Sun's Light is compounded of all the primary Colours mix'd in a due Proportion.

The Queries — and a wish

Newton ended the later editions with a celebrated set of open “Queries” — questions, often rhetorical, ranging far beyond optics into heat, chemistry, electricity and the cause of gravity. The one most consequential for optics proposed that light is corpuscular:

Are not the Rays of Light very small Bodies emitted from shining Substances?

This particle picture, set against Huygens's waves, would dominate for a century — until Young and Fresnel revived the wave theory, and the quantum photon much later restored a particulate aspect on entirely new terms. In a wary advertisement dated 1 April 1704, written the year after the death of his old rival Robert Hooke, Newton at last released the work he had long withheld:

I have here publish'd what I think proper to come abroad, wishing that it may not be translated into another Language without my Consent.

I. N. · April 1, 1704