Faraday effect
The Faraday effect or Faraday rotation is an interaction between light and a magnetic field. The rotation of the plane of polarization is proportional to the intensity of the component of the magnetic field in the direction of the beam of light.
The Faraday effect, also called the Magneto-Optic Effect, discovered by Michael Faraday in 1845, was the first experimental evidence that light and magnetism are related. This effect occurs in most optically transparent dielectric materials when they are subject to strong magnetic fields.
The Faraday effect is a result of ferromagnetic resonance when the permeability of a material is represented by a tensor. This resonance causes waves to be decomposed into two circularly polarized rays which propagate at different speeds, a property known as circular birefringence. The rays can be considered to re-combine upon emergence from the medium, however owing to the difference in propagation speed they do so with a net phase offset, resulting in a rotation of the angle of linear polarization.
The relation between the angle of rotation of the polarization and the magnetic field in a diamagnetic material is:
β =  VB d
where β is the angle of rotation (in radians)
B is the magnetic flux density in the direction of propagation (in teslas)
d is the length of the path (in metres) where the light and magnetic field interact
Then V is the Verdet constant for the material. This empirical proportionality constant (in units of radians per tesla per metre) varies with wavelength and temperature and is tabulated for various materials.
A positive Verdet constant corresponds to L-rotation (anticlockwise) when the direction of propagation is parallel to the magnetic field and to R-rotation (clockwise) when the direction of propagation is anti-parallel. Thus, if a ray of light is passed through a material and reflected back through it, the rotation doubles.
One of the most familiar optical instruments utilizing this effect is the Faraday rotator; one well-know present-day application is in the protective device used to prevent the destruction of high- power laser system by backreflections from the target or other ” downstream ” system points.
There are a few applications of Faraday rotation in measuring instruments. For instance, the Faraday effect has been used to measure optical rotatory power, for amplitude modulation of light, and for remote sensing of magnetic fields.
Also known as Kundt effect or magnetic rotation.