The Faraday effect, also called Faraday rotation, occurs when the direction of polarization of an electromagnetic wave is changed when the wave passes through a piece of transparent material permeated by a magnetic field. The amount of the rotation is determined by a property of the transparent material called its Verdet coefficient, by the thickness of the piece of material, and by the strength of the magnetic field. The law is usually expressed as θ = VBL, where V is the Verdet coefficient of the material, B is the strength of the magnetic field, and L is the thickness of the piece of material.
The Faraday effect should not be confused with Faraday’s law, which describes the induction of current in a wire loop by a changing magnetic field inside the loop. The two phenomena were both described by English physicist Michael Faraday (1791–1867) in the nineteenth century, but they are distinct.
Faraday discovered the Faraday effect in 1845 while systematically investigating the interactions of magnetic fields, transparent materials, and polarized light (light is a form of electromagnetic radiation). The electromagnetic nature of light was not yet understood, and Faraday’s observation was the first to show that light is related to magnetism: as Faraday noted in his diary, “there was an effect produced on the polarized ray, and thus magnetic force and light were proved to have relation to each other. This fact will most likely prove exceedingly fertile and of great value in the investigation of both conditions of natural force.”
The causes of the Faraday effect lie in the detailed physics of the transmission of electromagnetic radiation through material objects; the Faraday effect does not appear when electromagnetic rays pass through a magnetic field in a vacuum. In brief, a linearly polarized ray can be thought of as the superposition or simultaneous presence of two rays circularly polarized in opposite directions. In a medium with nonzero Verdet coefficient, these two circularly polarized rays experience different delays (for complex reasons). When they emerge from the slab of material they recombine into a single, linearly-polarized ray whose direction of polarization has been rotated through some angle θ.
The Faraday effect has practical uses. For instance, it can be used to detect the presence of a magnetic field, which in turn can be used to detect the presence of an electrical current. There are other methods for detecting magnetic fields, but the Faraday effect can be used to detect faint fields and fields in spaces that are difficult to access, such as super-hot plasmas. The Faraday effect is also being investigated for use in optical computing. By changing the value of a magnetic field rapidly, the polarization of a laser beam can be rapidly modulated (changed in a controlled way). This can be used to impress information on the laser beam, and that information can be detected by a receiving device. For detection, the polarized light could be passed through a polarizing filter, which would block the light more or less depending on its rapidly-changing angle of polarization. (A similar effect occurs when a pair of Polaroid sunglasses blocks glare from the sky or from reflective surfaces. By rotating a pair of Polaroid sunglasses, you can observe the changing brightness of different sources of polarized light.) A relatively simple brightness detector could then record the data signal. Such devices have been proposed as ultra-high-speed data links connecting computing elements.