奥古斯丁·菲涅耳Augustin-Jean Fresnel

奥古斯丁·菲涅耳的父母是Jacques Fresnel和Augustine Mérimée。Jacques Fresnel是一位建筑师，承担了重大建筑工程。1785年，他受雇于Victor-François de 路易·德布罗意，即路易十五和路易十六时期的第二公爵和法国元帅，对他的城堡进行重大改进。正是在他从事这个项目时，他遇到了Augustine Mérimée，她是de 德布罗意庄园管家的女儿，两人结婚了。Jacques Fresnel在他的儿子菲涅耳出生时仍在de 德布罗意的庄园工作。在de 德布罗意的城堡工作完成后，Jacques和Augustine带着他们的儿子菲涅耳去了瑟堡，Jacques在那里受雇于港口建设。

我们应该注意到，Jacques和Augustine 菲涅耳是詹森主义者；也就是说，他们是Cornelius Otto Jansen（1585-1638）的追随者，Jansen领导了以他命名的罗马天主教改革运动。Jansen认为，人不能通过自己的行为获得救赎，因为谁将被基督引向永生，即少数选民，谁注定被诅咒，即大众，都是预先注定的。菲涅耳在严格的詹森主义价值观和严厉的氛围中长大，这对他余生产生了强烈影响。

法国大革命始于1789年7月14日攻占巴士底狱，当时菲涅耳一岁。路易十六于1793年1月21日被处决，随后是恐怖统治时期。1794年，当菲涅耳六岁时，法国的政治局势如此艰难，以至于瑟堡港口的建设工作不得不停止。菲涅耳一家去了卡昂以北的村庄马蒂厄。在那里，年幼的菲涅耳度过了他童年余下的岁月，也是在那里，他的父母为他提供了初等教育。没有记录显示这个年幼男孩在这个阶段有任何教育成就，而且，虽然这样说对他的父母可能有些苛刻，但看来他们完全未能发掘出他们儿子的才能。

十二岁时，菲涅耳 开始在卡昂的中央学校学习。在这里，他第一次接触到科学，并开始表现出对数学的喜爱，尤其是由于一些出色的教学。当时 菲涅耳 对自己的未来职业几乎没有怀疑，因为他坚定地打算从事工程。他具备从事这种职业所需的兴趣、技能和背景，正是怀着这一想法，他于1804年进入巴黎综合理工学院。在那里学习两年后，他进入桥梁与道路学校，三年后完成课程，随后取得土木工程师资格。之后，他受雇于桥梁与道路工程队，并被派往旺代。1804年，拿破仑在旺代地区中心建立了军事和行政城镇永河畔拉罗什。该镇建有宽阔的矩形街道、营房和一个大型阅兵场。菲涅耳 受雇参与一项道路建设计划，该计划旨在将这座城镇与旺代其他地区连接起来。

另一个法国重大工程项目是修建一条贯穿法国、连接西班牙与意大利北部的主要道路。菲涅耳 于1812年开始参与这一项目，当时他驻扎在尼永，但他已经在业余时间从事科学工作。令 菲涅耳 着迷的一个主题是光，他于1814年中期开始进行实验。1815年3月1日，曾被流放到厄尔巴岛的拿破仑带着一些卫兵在戛纳登陆。菲涅耳 对这一事态转变如此不安，以至于他离开了工程工作，并提出为国王而战、对抗拿破仑。到3月20日，许多军队已加入拿破仑，他已抵达巴黎。当然，这意味着 菲涅耳 使自己陷入了困境，结果他失去了工程职位，并受到警察监视。菲涅耳 几乎没有别的选择，只能回到 Mathieu 的家中，他这样做了。

事实上，各种情况凑在一起，给了 菲涅耳 所需的空闲时间，使他能专注于光的实验。在此期间，他在光学方面的工作使他确信光的波动说的正确性，而当时这一理论完全被抛弃，人们支持微粒说。拿破仑在滑铁卢战败后，菲涅耳 被恢复到他原来的工程职位。此后，他用于研究光的时间更少了，只能在假期进行。他被调到雷恩的一个工程职位，但不断请求休假，以便能去巴黎继续他的科学研究。

通过将数学分析应用于他的工作，菲涅耳 消除了对光的波动说的许多反对意见。他的许多早期工作是在不了解其他科学家最新贡献的情况下进行的。他既不知道 克里斯蒂安·惠更斯、莱昂哈德·欧拉 和 Young 提出的波动理论，也不知道大多数科学家支持的微粒说的最新发展。菲涅耳 从进行衍射实验开始，并取得突破：他将一张黑纸贴在衍射器的一边，观察到阴影内的亮带随之消失。由此他正确推断出，这些亮带是由来自衍射器两边边缘的光产生的；但由于阴影外的亮带仍然存在，他推断它们必定是由只从衍射器一边边缘反射的光产生的。

他能够计算出一些公式，这些公式根据振动同相和反相的位置给出明线和暗线的位置。1815年10月，他发表了他的第一篇关于光的波动理论的论文，并首次尝试解释衍射现象。随后，他使用那些适用于他的衍射实验的相同数学公式，对通过用两面镜子反射光源获得的干涉图样给出了理论结果。他通过实验验证了理论结果。在这个阶段，他所进行的相当类似的研究，正是托马斯·杨于1797年至1799年间在剑桥所进行的，但菲涅耳接下来通过基于一种新的数学表述发展出一种理论，向前推进到了一种新的理解。

他提出了这样的想法[1]：-

……在波前弧线上经过衍射体的每一点都会产生元波，并相互干涉。问题是要确定所有到达衍射体后方任意点的小波所产生的合振动。数学上的困难是巨大的，而解决它需要付出数月的努力。

菲涅耳于1816年7月发表了他最初的试探性结果，但要求他文章的读者保持耐心，因为他正在推导数学的进一步结果。在1817年研究光的偏振一段时间后，特别是反射对偏振光的影响，当Académie des Sciences宣布1819年的大奖将授予关于衍射的最佳作品时，他又回到了他的衍射理论。这对菲涅耳来说是一个把他的革命性工作呈现给世界的绝佳机会，他对自己的理论非常有信心，因为他从一个简单假设出发的数学推导所得出的结果，已经通过实验验证，理论与实验证据之间高度精确地吻合。他在提交截止前不久完成了他的数学工作，这使他能够使用后来被称为菲涅耳积分的方法计算衍射体后方每一点的光强。

1819年，评审Académie des Sciences大奖的委员会开会审议菲涅耳的提交作品，弗朗索瓦·阿拉戈担任主席，成员包括西莫恩·德尼·泊松、让-巴蒂斯特·毕奥和皮埃尔·西蒙·拉普拉斯。这个委员会对光的波动说并不友好，大多数人相信微粒模型。然而，西莫恩·德尼·泊松被菲涅耳提出的数学模型所吸引，并成功计算了其中一些积分，以找出超出菲涅耳所推导结果的进一步结果。西莫恩·德尼·泊松写道[3]：-

让平行光照射到一个不透明圆盘上，周围完全透明。圆盘当然会投下阴影——但阴影的正中心却是亮的。简言之，在不透明圆盘后方沿中心垂直线的任何地方都没有黑暗（紧贴圆盘后方除外）。

这是一个非凡的预言，但弗朗索瓦·阿拉戈要求对西莫恩·德尼·泊松基于菲涅耳数学模型所作的预言进行检验。果然，亮斑正如菲涅耳理论所预言的那样出现了。弗朗索瓦·阿拉戈在向Académie des Sciences[3]提交的关于菲涅耳参评该奖的报告中写道：-

你们的一位评审委员M 西莫恩·德尼·泊松从[菲涅耳]所报告的积分中推导出一个奇特的结果：当光线以仅稍微更倾斜的入射角穿透那里时，不透明圆形屏的阴影中心必定会被照亮，就像屏不存在一样。这一推论已提交直接实验检验，而观测完全证实了计算。

菲涅耳被授予大奖，他的工作为光的波动说提供了强有力的论据。然而，由反射产生的光的偏振仍然为微粒说提供了强有力的论据，因为波动说从未对此作出过解释。菲涅耳和弗朗索瓦·阿拉戈此时非常自信能够用菲涅耳的理论解释这一效应，便着手对偏振进行进一步研究，菲涅耳发现了后来被称为圆偏振光的东西。除了光是横波之外，没有任何假说能导致所获得的实验结果，于是在1821年，菲涅耳发表了一篇论文，在文中他确信地宣称光是横波。

菲涅耳已经使许多人皈依了光的波动理论，甚至包括那些先前相信微粒理论的人中最狂热的一些人，但他关于光是横波的断言对大多数人来说还是太过分了。甚至弗朗索瓦·阿拉戈也不同意这一主张，但当菲涅耳接下来表明双折射可以从横波假设推导出来时，他使批评者大为震惊。

[2]的作者写道：-

尽管他在光学方面的工作生前很少得到公众认可，菲涅耳坚持认为，即使是杰出同行的赞誉，也无法与发现一个理论真理或用实验证实一项计算的乐趣相比。

1824年后，他用于光研究的时间减少了。他受雇于灯塔委员会，作为其工作的一部分，他发展了在灯塔中使用复合透镜代替反射镜的方法。对于这项工作[1]：-

……他带来了先前在科学工作中表现出的同样的创造性、专注和毅力。

1823年，菲涅耳当选为Académie des Sciences院士。他还当选为伦敦皇家学会会士，并于1827年获得该学会的伦福德奖章。

菲涅耳于1827年因肺结核去世，享年39岁。他一生都在与病弱作斗争，但值得注意的是，尽管遭受严重疲劳，他仍能承担异常繁重的工作。也许正是父母严格的宗教教养赋予了他长期战胜疾病的力量。他认为[1]：-

……个人成就、履行职责和服务社会的最高美德。严肃、专注、被早逝的念头所困扰，菲涅耳将自己紧密地束缚于这些理想，回避享乐和娱乐，工作到精疲力竭。尽管他所尝试的一切都紧迫，菲涅耳总是注重细节、有条不紊且彻底。在科学上如同在政治上一样，他顽强地坚持自己的信念，并以勇气和活力为之辩护。……当他人的行为达不到他自己的高标准道德时，他会表达愤怒。有时这近乎于一种令人怨恨的自以为是，但总的来说，同时代人认为他矜持、温和且仁慈。

Augustin Fresnel's parents were Jacques Fresnel and Augustine Mérimée. Jacques Fresnel was an architect who undertook major building works. In 1785 he was employed by Victor-François de Broglie, the Second Duke and marshal of France under Louis XV and Louis XVI, to undertake major improvements on his château. It was while he was working on this project that he met Augustine Mérimée the daughter of the overseer of de Broglie's estate and the two were married. Jacques Fresnel was still working on de Broglie's estate when his son Augustin was born. After work on de Broglie's château had been completed Jacques and Augustine, with their son Augustin, went to Cherbourg where Jacques was employed on the construction of the harbour.

We should note that Jacques and Augustine Fresnel were Jansenists; that is they were followers of Cornelius Otto Jansen (1585-1638) who led the Roman Catholic reform movement named after him. Jansen argued that men cannot achieve salvation through their actions since it is predestined who Christ will lead to eternal life, the select few, and who are doomed to damnation, the multitude. Augustin was brought up with strict Jansenist values in a stern atmosphere which would strongly influence him for the rest of his life.

The French Revolution began with the storming of the Bastille on 14 July 1789 when Augustin was one year old. Louis XVI was executed on 21 January 1793 and there followed the reign of terror. In 1794, when Augustin was now six years old, the political situation in France was so difficult that the construction work on the harbour at Cherbourg had to be halted. The Fresnel family went to Mathieu, a village north of Caen. There young Fresnel spent the rest of his childhood years, and there his elementary education was provided by his parents. There is no record of any educational achievements by the young boy at this stage and, although it may be a little harsh on his parents to say so, it appears that they completely failed to bring out their son's talents.

At age twelve Fresnel began his studies at the École Centrale in Caen. Here he was first introduced to science and he began to show a liking for mathematics, particularly as a result of some fine teaching. There was little doubt in Fresnel's mind at this time regarding his future career for he was firmly set on engineering. He had the right interests, skills, and background for such a career and it was with this in mind that he entered the École Polytechnique in Paris in 1804. After two years there he entered the École des Ponts et Chaussées, completing the course there in three years after which he was qualified as a civil engineer. He was then employed by the Corps des Ponts et Chaussées who sent him to Vendée. In 1804 Napoleon had established the military and administrative town of La Roche-sur-Yon in the centre of the Vendée region. The town was built with wide rectangular streets, barracks and a large parade ground. Fresnel was employed on a programme of road building which was designed to link this town with the rest of Vendée.

Another major French engineering project was the building of a major road through France connecting Spain with northern Italy. Fresnel began working on this project in 1812 when he was based in Nyon but already he was undertaking scientific work in his spare time. One topic which fascinated Fresnel was that of light and he began to undertake experiments in the middle of 1814. On 1 March 1815 Napoleon, who had been exiled on Elba, landed at Cannes with some of his guards. Fresnel was so upset by this turn of events that he left his engineering job and offered to fight for the King against Napoleon. By 20 March many troops had joined Napoleon and he had reached Paris. Of course this meant that Fresnel had put himself in a difficult position, and as a consequence he lost his engineering post and was put under police surveillance. Fresnel had few options left but to return to his home in Mathieu and this he did.

In fact circumstances had conspired to give Fresnel the free time he needed to concentrate on his experiments with light. During this period his work on optics convinced him of the validity of the wave theory of light which was, at that time, totally discarded in favour of the corpuscular theory. After Napoleon was defeated at Waterloo, Fresnel was reinstated into his old engineering appointment. After this he had less time for his research on light which he was only able to undertake in his vacations. He was transferred to an engineering post in Rennes but continually requested leave so that he could go to Paris to continue his scientific investigations.

By applying mathematical analysis to his work Fresnel removed many of the objections to the wave theory of light. Much of his initial work was undertaken without knowledge of the latest contributions by other scientists. He neither knew of the wave theories that had been postulated by Huygens, Euler and Young, nor did he know of the latest developments in the corpuscular theory supported by the majority of scientists. Fresnel began by undertaking experiments with diffraction and made a breakthrough when he attached a piece of black paper to one edge of a diffracter and observed that then the bright bands within the shadow vanished. From this he correctly deduced that these bright bands were produced by light coming from both edges of the diffracter but since bright bands outside the shadow remained he deduced that they must result from light reflected from only one edge of the diffracter.

He was able to calculate formulae which gave the position of the bright and dark lines based on where the vibrations were in phase and where they were out of phase. He published his first paper in October 1815 on his wave theory of light and made a first attempt to explain the phenomenon of diffraction. He then used his same mathematical formulae which worked for his diffraction experiments to give theoretical results on interference patterns obtained by reflecting a light source with two mirrors. He verified the theoretical results by experiment. At this stage he had carried out fairly similar investigations that Thomas Young had carried out between 1797 and 1799 in Cambridge, but Fresnel next moved forward to a new understanding by developing a theory based on a new mathematical formulation.

He put forward the idea that [1]:-

... elementary waves arise at every point along the arc of the wave front passing the diffracter and mutually interfere. The problem was to determine the resultant vibration produced by all the wavelets reaching any point behind the diffracter. The mathematical difficulties were formidable, and a solution was to require many months of effort.

Fresnel published his first tentative results in July 1816 but asked that the readers of his article show patience while he worked out further consequences of the mathematics. After working for a while on polarisation of light during 1817, in particular the influence of reflection on polarised light, he returned to his theories of diffraction when the Académie des Sciences announced that the Grand Prix for 1819 would be awarded to the best work on diffraction. It was a great chance for Fresnel to put his revolutionary work before the world and he was very confident of his theory since his mathematical deductions from the one simple hypothesis led to results which he had verified experimentally giving a highly accurate agreement between theory and experimental evidence. He completed his mathematical work just before the time for submission and this allowed him to calculate the intensity of light at every point behind the diffracter using what were later called Fresnel's integrals.

In 1819 the committee to judge the Grand Prix of the Académie des Sciences, with Arago as chairman, and including Poisson, Biot and Laplace, met to consider Fresnel's submission. It was a committee which was not well disposed to the wave theory of light, most believing in the corpuscular model. However Poisson was fascinated by the mathematical model which Fresnel proposed and succeeded in computing some of the integrals to find further consequences beyond those which Fresnel had deduced. Poisson wrote [3]:-

Let parallel light impinge on an opaque disk, the surrounding being perfectly transparent. The disk casts a shadow - of course - but the very centre of the shadow will be bright. Succinctly, there is no darkness anywhere along the central perpendicular behind an opaque disk (except immediately behind the disk).

This was a remarkable prediction, but Arago asked that Poisson's predictions based on Fresnel's mathematical model be tested. Indeed the bright spot was seen to be there exactly as Fresnel's theory predicted. Arago stated in his report on Fresnel's entry for the prize to the Académie des Sciences [3]:-

One of your commissioners, M Poisson, had deduced from the integrals reported by [Fresnel] the singular result that the centre of the shadow of an opaque circular screen must, when the rays penetrate there at incidences which are only a little more oblique, be just as illuminated as if the screen did not exist. The consequence has been submitted to the test of direct experiment, and observation has perfectly confirmed the calculation.

Fresnel was awarded the Grand Prix and his work was a strong argument for a wave theory of light. However polarisation of light produced by reflection still provided a strong argument in favour of the corpuscular theory, since no explanation from a wave theory had ever been made. Fresnel and Arago, now very confident that they could explain this effect with Fresnel's theory, undertook further work on polarisation and Fresnel discovered what was later called circularly polarised light. No hypothesis led to the experimental results obtained other than that light is a transverse wave and, in 1821, Fresnel published a paper in which he claimed with certainty that light is a transverse wave.

Although Fresnel had made many converts to the wave theory of light, even from the most ardent of those previously believing in the corpuscular theory, his assertion that light is a transverse wave was a step too far for most. Even Arago dissented from this claim but Fresnel stunned his critics when he next showed that double refraction could be deduced from the transverse wave hypothesis.

The author of [2] writes:-

Although his work in optics received scant public recognition during his lifetime, Fresnel maintained that not even acclaim from distinguished colleagues could compare with the pleasure of discovering a theoretical truth or confirming a calculation experimentally.

After 1824 he devoted less time to his researches on light. He was employed by the Lighthouse Commission and as part of his effort he developed the use of compound lenses instead of mirrors for lighthouses. To this work [1]:-

... he brought the same inventiveness, concentration, and perseverance previously manifest in his scientific work.

In 1823 Fresnel was elected to the Académie des Sciences. He was also elected to the Royal Society of London and he received its Rumford Medal in 1827.

Fresnel died of tuberculosis in 1827 at the age of 39. He had struggled throughout his life against ill health but it is remarkable that he was able to undertake an exceptionally high workload despite suffering from severe fatigue. Perhaps it was the strict religious upbringing by his parents which gave him the strength to overcome his illness for so long. He saw [1]:-

... the highest merit in personal achievement, performance of duty, and service to society. Serious, intent, haunted by thoughts of an early grave, Fresnel bound himself closely to these ideals, shunning pleasures and amusements and working to the point of exhaustion. Despite the urgency of everything he attempted, Fresnel was always attentive to detail, systematic, and thorough. In science no less than in politics he held tenaciously to his convictions and defended them with courage and vigour. ... he voiced outrage when the behaviour of others fell short of his own high ethical standards. At times this approached a rankling self-righteousness, but generally his contemporaries saw him as reserved, gentle, and charitable.