lecture11 - Lecture 11 Notes 07 18 Polarization and...

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Lecture 11 Notes: 07 / 18 Polarization and polarizing filters Light and other EM radiation is an oscillating EM wave. As we mentioned before, the electric field for an individual wave points in a particular direction perpendicular to the wave's motion, while the magnetic field is perpendicular both to the wave's motion and to the electric field. The direction that the electric field points is called polarization . Non-polarized light contains waves polarized in all directions, randomly mixed together. If every wave is is polarized in the same direction, then the light is said to be polarized. Suppose we have a non-polarized radio wave. We can pass it through a contraption consisting of closely spaced conducting wires, all pointing in the same direction: When a radio wave with random polarization passes through the wires, the vertical component of the electric field, which is aligned with the wires, will drive current up and down the wires, and will thus lose its energy to the resistance and dissipate. The horizontal component, however, will pass through unimpeded, as it cannot drive the electric charges side to side. The remaining wave will thus be polarized in the horizontal direction (we will call this the direction of the polarizing filter). Its amplitude will be the same as that of its original horizontal component. If the wave was originally polarized at an angle φ to the direction of the filter, and had amplitude E 0 , its new amplitude will be the size of its original component that was aligned along the filter: The important thing is that the wires must be very close together compared to the wavelength, otherwise the waves will be able to go right through the gaps without causing current to flow, and we'll have a diffraction grating rather than a polarizing filter. We can construct a similar polarizing filter for light, but instead of parallel wires, we must use a material with a similar type of microscopic structure, that will admit only EM waves polarized in a particular direction. Such materials (various types of polymers or crystals) are quite cheap and easily available.
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Note that an electromagnetic wave perpendicular to the filter cannot pass through at all, since in this case cos φ = 0, and thus the transmitted wave's amplitude is also 0. This is the principle behind 3D glasses. The image on the screen is a superposition of two polarized images: one for the left eye, polarized in the vertical direction, and another for the right, polarized in the horizontal direction. The left side of the glasses is a vertical polarizing filter, and so it only admits the image for the left eye, while the right side is a horizontal polarizing filter, and only admits the image for the right eye. Since depth perception is primarily based on the difference of images between the two eyes, the 3D image creates a perception of the distance of the objects on the screen from the viewer. The
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This note was uploaded on 09/19/2011 for the course PHYS 1C taught by Professor Smith during the Spring '07 term at UCSD.

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lecture11 - Lecture 11 Notes 07 18 Polarization and...

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