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What is polarization? What does it have to do with the cosmic microwave background?

Stephen Smith, Charlotte, North Carolina
Light patterns
Light is a wave. Like a wave on the ocean, it represents an oscillation — though not a wave of the water’s surface, but of the electromagnetic field that permeates space. In physics, we say a wave is “polarized” if its oscillation has a preferred orientation. A water wave, for example, is always up-down-up-down and never left-right-left-right; such a wave is “100 percent linearly polarized.”

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