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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. The theoretical basis for that relation, now called electromagnetic radiation, was developed by James Clerk Maxwell in the 1860's and 1870's. This effect occurs in most optically transparent dielectric materials (including liquids) 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.

Faraday rotation caused by the earth's magnetic field is one phenomenon that affects the polarization of radio waves propagating through the atmosphere.

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.

The relation between the angle of rotation of the polarization and the magnetic field in a diamagnetic material is:

Where β is the angle of rotation (in minutes of arc).
B is the magnetic flux density in the direction of propagation (in gauss).
d is the length of the path (in cm) where the light and magnetic field interact.
Then is the Verdet constant for the material. This empirical proportionality constant (in units of minutes of arc per gauss per cm of path, or in SIThe International System of Units (symbol: SI (for the French phrase Systeme International d'Unites , is the most widely used system of units. It is used for everyday commerce in virtually every country of the world except the United States, and it is uni units, 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.

Some materials, such as terbium gallium garnet (TGG) have extremely high Verdet constants (~ -40 radIn mathematics and physics, the radian is a unit of angle measure. It is the SI derived unit of angle. It is defined as the angle subtended at the center of a circle by an arc of circumference equal in length to the radius of the circle. Angle measures in TTesla can refer to: Scientist and inventor Nikola Tesla The tesla, an SI unit named after Nikola Tesla. Tesla, a band named after Nikola Tesla.^-1 mFor other uses of "metre" and "meter", see Metre (disambiguation). The metre is the basic unit of length in the International System of Units (SI: Systeme International d'Unites). It is defined as the length of the path travelled by light in absolute vacu^-1). By placing a rod of this material in a strong magnetic field, Faraday rotation angles of over 45° can be achieved. This allows the construction of Faraday rotatorA Faraday rotator is an optical device that rotates the polarization of light due to the Faraday effect. The Faraday rotator works because one polarization of the input light is in ferromagnetic resonance with the material which causes its phase velocitys, which are the principle component of Faraday isolatorA Faraday isolator or optical isolator is an optical component which allows the transmission of polarised light in only one direction. They are typically used to prevent unwanted feedback into an optical oscillator (A laser cavity is a good example. The os, devices which transmit light in only one direction.

Similar isolators are constructed for microwave systems by using ferriteFerrites are ferromagnetic ceramic materials, compounds of iron, boron and barium or strontium or molybdenum. Ferrites have a high magnetic permeability, which allows them to store stronger magnetic fields than iron. Ferrites are often produced as powder, rods in a waveguide with a surrounding magnetic field.