Physics 1690

Treatise on Light

Christiaan Huygens

Light is a wave, and every point it touches launches a fresh ripple — enough to explain how light reflects and bends.

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In depth · the introduction

Drop two pebbles in a pond and the ripples cross, pass through each other, and carry on unchanged. Huygens said light does exactly that — and from that one picture he explained how it reflects and bends.

The big idea

In Huygens' day no one agreed what light was. He proposed that it is a wave — a spreading disturbance — travelling through an invisible “ether” that fills all space, the way sound travels through air.

His key move is almost childishly simple and astonishingly powerful. Picture a wave as a moving front. Now treat every single point along that front as if it were a tiny new source, sending out its own little ripple. A moment later, the new wavefront is just the smooth surface that touches all those little ripples at once. Roll that rule forward and you can predict where any wave will go — and what happens when it hits a mirror or slips into glass.

How it came about

Huygens was already one of Europe's most admired scientists — he had discovered Saturn's rings and invented the pendulum clock. He worked out his theory of light in Paris around 1678 and read it aloud to the new Royal Academy of Sciences, but he sat on it for twelve years, publishing only in 1690, almost apologetically calling it something he had “written rather carelessly.”

Across the Channel, Isaac Newton was building the opposite case: light as a hail of tiny particles. Two giants, two pictures — and Newton's enormous prestige won the century. Huygens' waves were pushed into the shadows until experiments in the 1800s brought them roaring back.

Why it mattered

With nothing but his wavelet rule, Huygens explained on paper why mirrors reflect at equal angles and why a straw looks snapped where it enters water. And he drew a daring line in the sand: light, he insisted, must travel slower in glass and water than in air. Newton's particles demanded the opposite. For 160 years no one could tell who was right — until 1850, when Léon Foucault measured light slowing down in water. Huygens, dead since 1695, had been right all along.

A way to picture it

Think of a stadium wave. Each person, seeing a neighbour rise, stands up a beat later — and the wave sweeps around the arena even though nobody actually walks anywhere. Huygens' little ripples are those individual stand-ups; the wave you see is just their sum. Now imagine the people on one side of the stadium are told to react more slowly. The leading edge of the wave swivels and heads off in a new direction — which is exactly how light bends when it crosses into glass and slows down.

Where it sits

Huygens stands between the astronomy of Kepler and Newton and the modern physics of waves. His rival Newton — also in this Library — would rule optics with particles for a century. But the wave torch was carried on by Young and Fresnel, and finally by Maxwell, who showed that light is a wave of electric and magnetic fields. Then the wheel turned once more: Planck and Einstein found that light also arrives in particle-like packets. Huygens and Newton, it turned out, were each holding half of the answer.

The original document

Original source text

Preface

Christiaan Huygens · Traité de la Lumière · The Hague, 1690 (trans. S. P. Thompson, 1912)

I wrote this Treatise during my sojourn in France twelve years ago, and I communicated it in the year 1678 to the learned persons who then composed the Royal Academy of Science.

There will be seen in it demonstrations of those kinds which do not produce as great a certitude as those of Geometry, and which even differ much therefrom, since whereas the Geometers prove their Propositions by fixed and incontestable Principles, here the Principles are verified by the conclusions to be drawn from them; the nature of these things not allowing of this being done otherwise.

[I relate these particulars] not for the purpose of detracting from the merit of those who, without having seen anything that I have written, may be found to have treated of like matters: as has in fact occurred to two eminent Geometricians, Messieurs Newton and Leibnitz, with respect to the Problem of the figure of glasses for collecting rays when one of the surfaces is given.

The Hague · 8 January 1690

I · Rays propagated in straight lines

The successive movement of Light being confirmed in this way, it follows, as I have said, that it spreads by spherical waves, like the movement of Sound.

So it arises that around each particle there is made a wave of which that particle is the centre.

For although the particular waves produced by the particles comprised within the space CAE spread also outside this space, they yet do not concur at the same instant to compose a wave which terminates the movement, as they do precisely at the circumference CE, which is their common tangent.

[ … ]

[By Rømer's timings of Jupiter's satellites] the velocity of Light is more than six hundred thousand times greater than that of Sound. This, however, is quite another thing from being instantaneous, since there is all the difference between a finite thing and an infinite.

Another property of waves of light, and one of the most marvellous, is that when some of them come from different or even from opposing sides, they produce their effect across one another without any hindrance.

III · On refraction

The Sines of the angles … have a certain ratio between themselves; which ratio is always the same for all inclinations of the incident ray, at least for a given transparent body. This ratio is, in glass, very nearly as 3 to 2; and in water very nearly as 4 to 3; and is likewise different in other diaphanous bodies.

But let us suppose that it transmits this movement less quickly … Now all these circumferences have for a common tangent the straight line BN … It is then BN … which terminates the movement that the wave AC has communicated within the transparent body.

Mr. Fermat was the first to propound this property of refraction, holding with us, and directly counter to the opinion of Mr. Des Cartes, that light passes more slowly through glass and water than through air.

V · The strange refraction of Iceland crystal

Before finishing the treatise on this Crystal, I will add one more marvellous phenomenon which I discovered after having written all the foregoing.

For though I have not been able till now to find its cause, I do not for that reason wish to desist from describing it, in order to give opportunity to others to investigate it.