物理學 1704

光學：關於光的反射、折射、拐折與顏色的論著

艾薩克·牛頓

白光是多種顏色的混合；稜鏡只是把牠們分開，從不製造顏色。

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In depth · the introduction

牛頓拿來世上最尋常的東西——一束陽光——證明了牠暗地裡其實是一大群顏色，結伴而行。

核心想法

在陽光裡舉起一塊玻璃稜鏡，牆上便出現一條彩虹色帶，次序總是一樣。最省事的猜想，是玻璃給光染了色。牛頓證明的恰恰相反：那些顏色一直就在白光裡，稜鏡不過把牠們分開，因為每種顏色各按自己固定的角度被彎折——紫光最多，紅光最少。

他一錘定音的一招，是把彩虹再拼回去。用一片透鏡、或一個反方向放的稜鏡把散開的顏色接住，牠們便重新合成白色。白光並不純、也不單一；牠是所有顏色的混合。稜鏡不製造顏色——牠只是把顏色分揀開。

牠是如何誕生的

1660 年代中期瘟疫流行，年輕的牛頓被從劍橋遣回鄉下。他買了一塊玻璃稜鏡——據他自己說是在鄉間集市上買的——把房間弄暗，玩起了陽光。抓住他的謎題，是牆上那塊光斑的形狀：一個圓孔本該投出一個圓點，稜鏡卻把牠拉成了一道長長的彩色條紋。正是這樁怪事，把他引向了「白光是一種混合」的念頭。

他在 1672 年的第一篇科學論文裡公布了這個想法，迎來的卻是一片反對——其中最猛烈的來自羅伯特·虎克。牛頓厭惡爭吵到這般地步，以致此後近三十年幾乎不再碰光學。他終於在 1704 年出版《光學》，用平白的英文、寫成人人都能重做的一連串實驗——而且意味深長地，正是在虎克去世後的第二年。

牠為何重要

一旦你知道白光是固定顏色的混合，就能把任何光拆開來「讀」。這就是光譜學——牠成了科學最有力的工具之一，讓我們僅憑一團火焰、乃至太陽與遙遠恆星所發出的顏色，就辨認出其中的化學元素。同一個發現解釋了彩虹，給了顏色在物理學中的位置，也促使牛頓發明反射望遠鏡，以躲開普通透鏡把顏色抹糊的毛病。

一個可以想像的畫面

想想鋼琴上彈下的一個和弦。牠傳到你耳裡是一個聲音，其實卻是同時敲響的好幾個音——訓練有素的耳朵能把牠們一個個挑出來。白光就是那個和弦；稜鏡，就是把牠分成各個音的耳朵。把這些音再一起彈響，你聽到的是和弦；把顏色再合到一起，你得到的是白光。

牠的位置

笛卡兒早已算出彩虹的幾何，卻沒說清牠為何有顏色；牛頓補上了這個「為何」。在光的本性上，他最大的對手是克里斯蒂安·惠更斯，惠更斯 1690 年的波動說（本館亦有收錄）與牛頓那套「光是微小粒子」的圖像相抗。牛頓的權威，讓粒子說佔了上風一百年——直到托馬斯·楊的干涉條紋（1804，本館同樣收錄）復活了波動，菲涅耳給了牠數學，馬克士威更揭示光是一種電磁波。最後的轉折來自量子物理：牠發現光原來既是波、也是粒子——儘管現代的光子，與牛頓的微粒毫不相同。

The original document

Original source text

定義與公理

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——白光是複合的

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.

疑問集——以及一個心願

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