物理学 1704

光学：关于光的反射、折射、拐折与颜色的论著

艾萨克·牛顿

白光是多种颜色的混合；棱镜只是把它们分开，从不制造颜色。

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In depth · the introduction

牛顿拿来世上最寻常的东西——一束阳光——证明了它暗地里其实是一大群颜色，结伴而行。

核心想法

在阳光里举起一块玻璃棱镜，墙上便出现一条彩虹色带，次序总是一样。最省事的猜想，是玻璃给光染了色。牛顿证明的恰恰相反：那些颜色一直就在白光里，棱镜不过把它们分开，因为每种颜色各按自己固定的角度被弯折——紫光最多，红光最少。

他一锤定音的一招，是把彩虹再拼回去。用一片透镜、或一个反方向放的棱镜把散开的颜色接住，它们便重新合成白色。白光并不纯、也不单一；它是所有颜色的混合。棱镜不制造颜色——它只是把颜色分拣开。

它是如何诞生的

1660 年代中期瘟疫流行，年轻的牛顿被从剑桥遣回乡下。他买了一块玻璃棱镜——据他自己说是在乡间集市上买的——把房间弄暗，玩起了阳光。抓住他的谜题，是墙上那块光斑的形状：一个圆孔本该投出一个圆点，棱镜却把它拉成了一道长长的彩色条纹。正是这桩怪事，把他引向了「白光是一种混合」的念头。

他在 1672 年的第一篇科学论文里公布了这个想法，迎来的却是一片反对——其中最猛烈的来自罗伯特·胡克。牛顿厌恶争吵到这般地步，以致此后近三十年几乎不再碰光学。他终于在 1704 年出版《光学》，用平白的英文、写成人人都能重做的一连串实验——而且意味深长地，正是在胡克去世后的第二年。

它为何重要

一旦你知道白光是固定颜色的混合，就能把任何光拆开来「读」。这就是光谱学——它成了科学最有力的工具之一，让我们仅凭一团火焰、乃至太阳与遥远恒星所发出的颜色，就辨认出其中的化学元素。同一个发现解释了彩虹，给了颜色在物理学中的位置，也促使牛顿发明反射望远镜，以躲开普通透镜把颜色抹糊的毛病。

一个可以想象的画面

想想钢琴上弹下的一个和弦。它传到你耳里是一个声音，其实却是同时敲响的好几个音——训练有素的耳朵能把它们一个个挑出来。白光就是那个和弦；棱镜，就是把它分成各个音的耳朵。把这些音再一起弹响，你听到的是和弦；把颜色再合到一起，你得到的是白光。

它的位置

笛卡儿早已算出彩虹的几何，却没说清它为何有颜色；牛顿补上了这个「为何」。在光的本性上，他最大的对手是克里斯蒂安·惠更斯，惠更斯 1690 年的波动说（本馆亦有收录）与牛顿那套「光是微小粒子」的图像相抗。牛顿的权威，让粒子说占了上风一百年——直到托马斯·杨的干涉条纹（1804，本馆同样收录）复活了波动，菲涅耳给了它数学，麦克斯韦更揭示光是一种电磁波。最后的转折来自量子物理：它发现光原来既是波、也是粒子——尽管现代的光子，与牛顿的微粒毫不相同。

The original document

Original source text

定义与公理

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.

第一卷——白光是复合的

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.

疑问集——以及一个心愿

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