A polarized microwave emitter and a polarized microwave receiver face each other on a table. At first, the emitter and receiver are polarized in the same direction (up-down), and all the emitted signal gets received.
When a metal comb in inserted between them, with the teeth pointed down, the signal is blocked. This is because the microwaves are polarized in the same direction that the teeth are pointing, creating small currents in the metal that reflect the microwaves backwards. When the comb teeth are turned horizontally, the signal passes through undisturbed.
When the receiver is turned 90 degrees, no signal is received. However, when the comb is inserted at a 45 degree angle, some signal passes through. This is because light waves are made up of two perpendicular components that add up to form the polarization of the wave. When one component is knocked out, the wave changes polarization.
See the original video and more on MIT TechTV - http://techtv.mit.edu/videos/752
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