How do crossed polarizing filters block light?

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liquid crystal Figure 1: Arrangements of molecules.

Crossed polarizing filters block light because they are oriented with their polarization axes at a 90° angle to one another. A polarizing filter functions by selecting a single component of polarized light to pass through while absorbing all other components of the incoming waves.1

When two such filters are placed in a series and "crossed," the first filter restricts the light to a specific polarization direction. Because the second filter is rotated by 90°, the polarization passed by the first filter is precisely the orientation that the second filter is designed to block. Consequently, no light can pass through the assembly unless an optically active material—such as a liquid crystal—is placed between them to alter the light's polarization.1