Faraday effect
The Faraday effect describes the rotation of the polarization plane of a linearly polarized electromagnetic wave in a medium, if a magnetic flux exists in parallel to the propagation direction of the wave.
History
The Faraday effect was discovered by Michael Faraday 13 September 1845. It was interpreted as the first experimental evidence that light and magnetism are related. That light can be described as an electromagnetic wave was theoretically a few years later derived by James Clerk Maxwell.
Basics
Any linearly polarized wave can be polarized as a superposition of two circular waves of the same frequency are interpreted with opposite sense of rotation. Extends parallel to the direction of propagation of the magnetic field lines, different in the refractive index of many materials and nLinks nRechts and therefore the wavelengths. Are the corresponding formulas
It follows that the plane of polarization
Continues to rotate, when an oscillation period, ie the time T = 1 / f is passed.
Optics
The Faraday effect occurs in the area of magneto-optics at the passage of light to through a transparent medium. Most dielectric materials (including liquids ) show the effect when they are exposed to a strong magnetic field. The rotation of the polarization plane is the larger, the stronger the magnetic flux density.
Does the media even without an applied magnetic field influence on the polarization plane on, one speaks of optical activity.
The angle of rotation, about which rotates the plane of polarization is calculated as follows:
Is the length of the light path through the substance, the magnetic flux density and the Verdet constant. is the medium and the wavelength dependent.
A positive Verdet constant results in a mathematically negative rotation when the magnetic field is parallel to the direction of propagation. An anti- parallel magnetic field then leads to a mathematically positive rotation ( see figure). This means that for a shaft that passes through the medium twice in opposite direction (back and forth ), doubles the rotation. This is an important difference to the optical activity of the left - or right-handed substances that will be reversed when passing through the medium in opposite directions.
An important application of the Faraday effect, the optical isolator. For these and other applications materials may be used with high Verdet constant, such as glass doped with terbium and terbium -gallium- garnet.
Radio waves
The Faraday effect also occurs in the pre-magnetized ferrite, and is used in the frequency range of several GHz in the range of microwaves, for the construction of gyrators and the realization of the Faraday rotators. The rotation of the polarization plane of special forms of two-ports for the transformation of impedances can then be implemented in its dual impedances in high-frequency technology.
The Faraday rotator, which is in this case located in a waveguide, has two orthogonal standing waveguide outputs that inrunning wave can be achieved by changing the magnetic field through a control line in each case to one of the outputs switch, as shown in the adjacent figure.
Running through radio waves, the ionosphere, the motion of unbound electrons causes the earth's magnetic field to rotate the polarization plane. (see also the refractive index of the plasma ) The angle of rotation is always proportional to the square of the wavelength ( β = k · λ ²), and is therefore in the medium and short wave range overlooked. Over very long distances, but can be observed above 90 ° even at 500 MHz rotation. At the even higher frequencies of the broadcasting satellite but the angle already is so low that the vertical and horizontal polarization of the transmission channels is not changed significantly.