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'''This page is under construction. Beware of loose bricks.'''
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By expressing [[Christiaan Huygens|Huygens]]' principle of secondary waves and [[Thomas Young (scientist)|Young]]'s principle of [[interference (wave propagation)|interference]] in quantitative terms, and supposing that simple colors consist of ''[[sine wave|sinusoidal]]'' waves, Fresnel gave the first satisfactory explanation of [[diffraction]] by straight edges, including the first explanation of rectilinear propagation that would satisfy a modern physicist.{{r|darrigol-2012|p=205}} By further supposing that light waves are purely ''[[transverse wave|transverse]]'', he explained the nature of [[polarization]] and lack thereof, the mechanism of ''chromatic polarization'' (the colors produced when polarized light is passed through a slice of doubly-refractive crystal followed by a second polarizer), and the [[transmission coefficient|transmission]] and [[reflection coefficient]]s at a boundary between transparent [[isotropy|isotropic]] media (including [[Brewster's angle]]). Then, by generalizing the relationship between wave speed and polarization for [[calcite]], he accounted for the directions and polarizations of the refracted rays in [[birefringence|doubly-refractive]] crystals of the ''biaxial'' class (those for which Huygens' secondary wavefronts are not axisymmetric). The period between the first publication of his pure-transverse-wave hypothesis and the presentation of his solution to the biaxial problem was less than a year. Later, by allowing the reflection coefficient to be ''[[complex number|complex]]'', he accounted for the change in polarization due to [[total internal reflection]], as exploited in the [[Fresnel rhomb]]. Defenders of the established corpuscular theory could not match his quantitative explanations of so many phenomena on so few assumptions.
Fresnel's legacy is the more remarkable in view of his lifelong battle with [[tuberculosis]], to which he succumbed at the age of 39. Although he did not become a public celebrity in his short lifetime, he lived just long enough to receive due recognition from his peers, including (on his deathbed) the [[Rumford Medal]] of the [[Royal Society of London]]
Inevitably, after the wave theory of light was subsumed by [[James Clerk Maxwell|Maxwell]]'s [[electromagnetism|electromagnetic]] theory in the 1860s and '70s, Fresnel's contribution was somewhat obscured.
== Early life ==
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== Religious views ==
Fresnel's parents were [[Catholic Church|Roman Catholics]] of the [[Jansenism|Jansenist]] sect, characterized by an extreme [[Augustine of Hippo|Augustinian]] view of [[original sin]]. In the early home-schooling that the four boys received from their mother, religion took first place. In 1802, Mme Fresnel wrote to Louis concerning Augustin:
{{quote|I pray God to give my son the grace to employ the great talents, which he has received, for his own benefit, and for the God of all. Much will be asked from him to whom much has been given, and most will be required of him who has received most.{{r|kneller-1911|p=147}} }}
Augustin Fresnel indeed regarded his intellectual talents as a gift from God, and considered it his duty to use them for the benefit of others. Plagued by poor health, and determined to do his duty before death thwarted him, he shunned pleasures and worked himself to exhaustion.{{r|silliman-2008|p=166}} According to his fellow engineer Alphonse Duleau, who helped to nurse him through his final illness, Fresnel saw the study of nature as the study of the power and goodness of God. He placed virtue above science and genius. Yet in his last days he needed "strength of soul", not against death alone, but against "the interruption of discoveries… of which he hoped to derive useful applications."{{r|kneller-1911|p=148–9n}} Although Jansenism is considered [[heresy|heretical]] by the Roman Catholic Church, the brief article on Fresnel in the ''[[Catholic Encyclopedia]]'' (v.6, 1909) does not mention his Jansenism, but simply acknowledges his deep religiosity and "remarkable" sense of duty.{{r|brock-1909}}
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Also in 1825, Fresnel extended his fixed design by adding a rotating array outside the fixed array.{{r|tag-2017b}} Each panel of the rotating array refracted part of the fixed light from a horizontal fan into a narrow beam.
[[File:MuseeMarine-phareFresnel-p1000466.jpg|thumb|left|First-order rotating catadioptric Fresnel lens, made in 1870, on display at the ''[[Musée national de la Marine]]'', Paris. In this case the dioptric prisms (inside the bronze circles) and catadioptric prisms (outside) are arranged
To reduce the loss of light in the reflecting elements, Fresnel proposed to replace the mirrors with ''catadioptric'' prisms, through which the light would pass by two refractions and one [[total internal reflection]].{{r|levitt-2013|p=79–80}} The result was the lighthouse lens as we now know it. In 1826 he assembled a small model for use on the [[Canal Saint-Martin]],{{r|musee-2017}} but he did not live to see a full-sized version.
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<ref name=appleton-1861>D. Appleton & Co., "Sea-lights", ''Dictionary of Machines, Mechanics, Engine-work, and Engineering'', 1861, [https://archive.org/details/appletonsdiction02appl v.2].</ref>
<ref name=boutry-1948>G.-A. Boutry, "Augustin Fresnel: His time, life and work, 1788–1827, ''Science Progress'', v.36, no.144 (October 1948), pp. 587–604; [http://www.jstor.org/stable/43413515 jstor.org/stable/43413515].</ref>
<ref name=brock-1909>H.M. Brock, [[s:Catholic Encyclopedia (1913)/Augustin-Jean Fresnel|"Fresnel, Augustin-Jean"]], ''Catholic Encyclopedia'', 1907–12, v.6 (1909).</ref>
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[[Category:19th-century deaths from tuberculosis]]
[[Category:Infectious disease deaths in France]]
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