Editing Faraday effect (section)

==History==
[[Image:Faraday with glass bar crop2.jpg|thumb|upright|[[Michael Faraday]] holding a piece of glass of the type he used to demonstrate the effect of magnetism on polarization of light, c. 1857.]]
By 1845, it was known through the work of [[Augustin-Jean Fresnel]], [[Étienne-Louis Malus]], and others, that different materials are able to modify the direction of polarization of light when appropriately oriented,<ref name=horvath-thesis>{{cite book|last1=Horváth|first1=Gábor|title=Polarization Patterns in Nature - Imaging Polarimetry with Atmospheric Optical and Biological Applications|date=2003|publisher=Eötvös University|location=Budapest|url=https://arago.elte.hu/?q=node/11|access-date=15 June 2014}}</ref> making polarized light a very powerful tool to investigate the properties of transparent materials. Faraday firmly believed that light was an electromagnetic phenomenon, and as such should be affected by electromagnetic forces. He spent considerable effort looking for evidence of electric forces affecting the polarization of light through what are now known as [[electro-optic effect]]s, starting with decomposing electrolytes. 

Faraday then attempted to look for the effects of magnetic forces on light passing through various substances. After several unsuccessful trials, he happened to test a piece of "heavy" glass, containing equal proportions of [[silica]], [[Boric acid|boracic acid]] (B(OH)<sub>3</sub>) and [[lead oxide]], that he had made during his earlier work on glass manufacturing.<ref name=mansuripur>{{cite journal|last1=Mansuripur|first1=Masud|title=The Faraday Effect|journal=Optics and Photonics News|issue=10|pages=32–36|url=http://www.mmresearch.com/articles/article3/|access-date=15 June 2014}}</ref> Faraday observed that when a beam of polarized light passed through the glass in the direction of an applied magnetic force, the polarization of light rotated by an angle that was proportional to the strength of the force. He used a [[Nicol prism]] to measure the polarization. He was later able to reproduce the effect in several other solids, liquids, and gases by procuring stronger electromagnets.<ref name="crowther-1920">{{cite book |last1=Crowther |first1=James Arnold |url=https://archive.org/details/lifediscoverieso00crowrich |title=The life and discoveries of Michael Faraday |date=1920 |publisher=Society for promoting Christian knowledge |pages=[https://archive.org/details/lifediscoverieso00crowrich/page/n59 54]–57 |access-date=15 June 2014}}</ref>

The discovery is well documented in Faraday's daily notebook.<ref>
{{Cite book
  | last1 = Faraday
  | first1 = Michael
  | author-link = Michael Faraday
  | title = Faraday's Diary
  | publisher = George Bell and Sons, Ltd.
  | volume = IV, Nov. 12, 1839 - June 26, 1847
  | edition = Thomas Martin
  | date = 1933
  | location = London
  | isbn = 978-0-7503-0570-9
  | url=https://archive.org/details/faradaysdiarybei00fara_2
  }} The diary is indexed by Faraday's original running paragraph numbers, not by page. For this discovery see #7504, 13 Sept. 1845 to #7718, 30 Sept. 1845.</ref> On 13 Sept. 1845, in paragraph #7504, under the rubric ''Heavy Glass'', he wrote:
{{Blockquote|text={{omission}} '''BUT''', when the contrary magnetic poles were on the same side,  ''there was an effect produced on the polarized ray'',  and thus magnetic force and light were proved to have relation to each other. {{omission}}|author=Faraday|title=Paragraph #7504|source=Daily notebook}} <!-- I do not know better how to mark this up, but the emphasis on the word BUT is an attempt to reflect what Faraday wrote. In the published notebook, which is a facsimile of the original, one can see that Faraday wrote the word BUT very large, and underlined it several times. -->

He summarized the results of his experiments on 30 Sept. 1845, in paragraph #7718, famously writing:
{{Blockquote|text={{omission}} Still, I have at last succeeded in illuminating a magnetic curve or line of force, and in magnetizing a ray of light. {{omission}}|author=Faraday|title=Paragraph #7718|source=Daily notebook}}

=== Kerr experiments ===
Faraday's experimental methods were not sensitive enough, and the effect was only measured thirty years later by [[John Kerr (physicist)|John Kerr]].<ref name="crowther-1920" />

=== Later developments ===
In 1897, [[Henri Becquerel]] wrote the formula for the angle of rotation of the Faraday effect.<ref name=":0">{{Cite journal |last=Raman |first=C. V. |date=1945 |title=Centenary of the Faraday Effect |url=https://www.jstor.org/stable/24209321 |journal=Current Science |volume=14 |issue=11 |pages=281–283 |issn=0011-3891}}</ref>

In 1906, his son [[Jean Becquerel]], discovered a specific type of Faraday effect in paramagnetic materials.<ref name=":0" /> The dispersion curve was symmetric in paramagnetic materials and asymmetric in diamagnetic ones.<ref name=":0" />