# Why/how does a magnetic field affect polarized light?

**URL:** <https://boards.straightdope.com/t/why-how-does-a-magnetic-field-affect-polarized-light/135050>\
**Category:** Factual Questions\
**Created:** [November 3, 2002, 2:01am UTC](https://boards.straightdope.com/t/why-how-does-a-magnetic-field-affect-polarized-light/135050 "2002-11-03T02:01:24Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\
**Post date:** [November 3, 2002, 2:01am UTC](https://boards.straightdope.com/t/why-how-does-a-magnetic-field-affect-polarized-light/135050/1 "2002-11-03T02:01:24Z")

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I’m reading _Uncle Tungsten_ by Oliver Sacks, which is a sort of young-boy-discovers-the-joys-of-science autobiographical work. In one chapter, the author mentions in passing that a magnetic field will affect polarized light, but doesn’t go into details. Just what effect does a magnet have on polarized light? And why does it do so?

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**Author:** ![Luckie](https://avatars.discourse-cdn.com/v4/letter/l/dc4da7/32.png) [@Luckie](https://boards.straightdope.com/u/Luckie)\
**Post date:** [November 3, 2002, 3:04am UTC](https://boards.straightdope.com/t/why-how-does-a-magnetic-field-affect-polarized-light/135050/2 "2002-11-03T03:04:16Z")

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It’s called the Faraday effect.  
cheap link:  
[http://www.minidisc.org/eb/faraday\_effect.htm](http://www.minidisc.org/eb/faraday_effect.htm)  
-Luckie

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**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\
**Post date:** [November 5, 2002, 1:54pm UTC](https://boards.straightdope.com/t/why-how-does-a-magnetic-field-affect-polarized-light/135050/3 "2002-11-05T13:54:35Z")

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> [@](#):
>
> And why does it do so?

The magnetic field doesn’t directly affect the light. It affects the polarization currents in the material the light is passing through. With a magnetic field parallel to the direction of travel of the light, the charges will be displaced perpendicular to the plane of polarization, and will add a component of the other polarization. In a vacuum, the Farady rotation should be zero.

If the magnetic field is perpendicular to the polarization and propagation directions, the displacement due to the magnetic field is in the direction of travel, so this won’t affect the polarization. If the magnetic field were parallel to the electric field direction, no displacement due to the magnetic field would occur.

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**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\
**Post date:** [November 5, 2002, 2:21pm UTC](https://boards.straightdope.com/t/why-how-does-a-magnetic-field-affect-polarized-light/135050/4 "2002-11-05T14:21:56Z")

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**Luckie** and **ZenBeam** have already said the obvious thinhs I was gonna say, but just for the sake of completeness, the magnetic field affects the medium through which the light is passing, not the light itself. (since the light wave and the field are both electro-magnetic phenomena, an electric or magnetic field directly interacting with a photon would be required. The probability is non-zero, but _way_ low, and AFAIK has never been observed).

There are other magneto-optical effects related to polarization. The _Kerr Magnetic Effect_ (not to be confused with the more common Kerr Electro-Optic Effect) causes the polarization of light reflected from the polished pole face of an electromagnet.

I also note that when you place your radiating atoms in a magnetic field, you split the level degeneracy and the different lines have different polarizations, the \* Zeeman Effect\*