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Unformatted text preview: Argument from polarization that EM waves are transverse : We revisit now the transversal- ity of electromagnetic waves. Assume there were a longitudinal component to the E field. Consider now the light falling on two crossed polaroids. This longitudinal component may be transmitted through either polaroid, so that a longitudinal component would arrive to the screen through both polaroids even when they are orthogonal. Therefore, with a lon- gitudinal component the screen will never get totally dark when the two polaroids are in a plane perpendicular to the direction of propagation of the light. A variant of this argument is to consider the two polarizers rotating relative to each other in the plane orthogonal to the direction of propagation. As the brightness on the screen oscillates with the polarizers, one must conclude the electric field acts across its motion, i.e., transversally. Some textbooks explain the polarization effect by stretching a mechanical model too far. In such texts the electric field is modeled by a tension wave in a string that is passing through a fence with parallel beams. The mechanical wave naturally is transmitted only if the string oscillates parallel to the beams. This argument might imply the wrong component of the E field is transmitted. For this reason, those texts refer to the polarization direction of the polarizer, stating that perpendicular components are filtered out. But this argument may then persuade some students to believe a longitudinal component of the E field is also filtered out, such that this sort of argumentation, in addition to not empowering the students with an understanding of how simple dichroic devices work, also might prevent them from understanding a simple physical argument for why electromagnetic waves are transverse, and give the students a misleading physical picture of electromagnetic fields oscillating in the space between the wires....
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