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Light is, like radio waves, an electromagnetic vibration which travels through space . The amplitude of the vibration is, like waves in water, at right angles to the motion of the waves, but unlike water, the direction of the amplitude can be any direction at right angles to the movement, allowing a full 360° of possible directions . We take the direction of vibration as being the direction of the electric field (the magnetic field is at 90° to this) . Normal, unpolarized, light uses all of these directions, there is no preferred direction. Some natural materials, some angles of reflection, and some man-made substances will filter out light so as to permit the vibration of the electrical signal to be in one preferred direction . This effect is called polarization, and these materials are polarizers . The most striking demonstration of polarization is that once light has been polarized, passing it through a second polarizer can reduce the light amplitude to zero if the polarizer is turned so that its angle of polarization is at 90° to the angle of polarization of the light. Polaroid sunglasses exhibit this effect very noticeably, and they are supplied so as to reduce the amount of light that is reflected from horizontal surfaces (which is polarized quite strongly) so reducing glare . Polarization had already been discovered by 1678, but understanding of polarization as evidence of light waves was developed mainly in the period from 1800 onwards. Faraday' s discovery was that a beam of polarized light could have its angle of polarization altered by passing through a magnetic field, and since then, materials have been discovered which allow the effect to be greatly intensified. The material used in the Sony MD recording discs is terbium ferrite cobalt, an exotic compound which will alter the polarizing angle of light which is reflected from it depending on the extent to which the material is magnetized. The reflected beam can be passed to photodiodes, each with a polarizing sheet in front of it, so that the amount of light that enters each photodiode will depend on the angle of polarization of the beam. By making magnetization in one direction represent 0 and magnetization in the other direction represent 1, the relative amount of beam at the two photodiodes will alter for each change in magnetization, allowing digital signals to be obtained.
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