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Faraday Rotation

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Lab Topics:

Faraday Rotation

Interaction of Light, Matter, and Magnetic Fields

In 1845, Michael Faraday was searching for experimental evidence that the forces in nature were all interconnected. He made a remarkable discovery by carefully examining the polarization of light as it passed through a transparent material in the presence of a magnetic field. He observed that linearly polarized light propagating through matter parallel to a static magnetic field, experiences a rotation of the plane of polarization. The effect is small, but he was an exceptional experimenter and he unambiguously identified the phenomenon. The rotation of the plane of polarization is still called the "Faraday Rotation."

The Faraday rotation experiment appealed to TeachSpin as a "real" physics experiment to dramatically "show off" the capabilities of the Signal Processor/Lock-In Amplifier (SPLIA1-A). Faraday rotation seemed ideal because, in the presence of a magnetic field reasonably obtained with a laboratory power supply and a solenoid, there is only a small rotation of the plane of polarization.

Faraday rotation has a practical application in optical isolators. An optical isolator is a device that allows light to go through in one direction but severely attenuates reflected light propagating in the opposite direction. Modern ultra-high field permanent magnets and special paramagnetic glasses have made these devices quite small, but not cheap (about $2K). The configuration of the components is essentially the same as in FR1-A. The polarizers are set at 45°. The combined effect of the special glass and the large magnetic field rotates the polarization plane of the light 45° on each passage. A simple sketch will show how this works as an optical isolator.

Optical isolators have important applications in telecommunications preventing reflected signals on fiber optic cables from producing unwanted signals. Isolators are important when lasers are used because reflected light can cause havoc with the operation of the laser itself.

Although Michael Faraday discovered this effect in 1845, it wasn't modeled quantum mechanically until the 1960's. These theoretical calculations are too sophisticated for the undergraduate student, but an excellent simplified QM model is carefully presented in David Van Baak's AJP paper. (D.A. Van Baak, Resonant Faraday Rotation as a Probe of Atomic Dispersion, Am. J. Phys.64 (6) June 1996)