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Faraday rotation

(Faraday effect)
(rotation of optical polarization by a medium in a magnetic field)

Faraday rotation (or the Faraday effect) is a magnetohydrodynamics phenomena, of light polarization rotated when passing through a medium (such as plasma) in a magnetic field.

The amount of rotation is related to the strength of the magnetic field, thus is useful in the interpretation of observation. Rotation measure or RM refers to the inferred "strength" of rotation, which in astronomy (or some branches) it is commonly cited in units of radians per meter squared. Actual rotation of the polarization associated with a given RM is wavelength-dependent, the rotation in radians being the RM times the square of the wavelength in meters.