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In page Magneto-optic Kerr effect:

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where v p {\displaystyle v_{p}} is the velocity of light through the material, ε {\displaystyle \varepsilon } is the material permittivity, and μ {\displaystyle \mu } is the magnetic permeability; and thus the speed of light varies depending on its orientation. This causes fluctuations in the phase of polarized incident light.