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Faraday rotation (or the Faraday effect) is a rotation of light polarization that occurs as it passes through a medium (such as plasma) within a magnetic field. The rotation depends upon the electromagnetic radiation (EMR) displacing plasma particles of a given charge, typically its electrons, readily displaced due to their low mass. Within a plasma with equal free electrons and positrons (electron-positron plasma), opposing rotation-tendencies cancel.
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.
For radio sources, a minimum of the amplitude of the source's magnetic flux density (B) (specifically, the component of the vector in line with the observer) is directly related to the ratio of RM and the dispersion measure.
Under the right precise conditions, Faraday rotation can rotate EMR's polarization back to unpolarized.