why do some windows (especially car windows) have a strange dark and light alternating pattern when wearing polarized sunglasses?
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Because they themselves are laminated and that causes light reflected from them to be polarized, which in turn creates the interference pattern you see through another layer of polarizing film in the sunglasses.
For a fun trick try looking at your cell phone, or even a calculator or other digital display and rotate the device to see similar effects of cross-polarization.
The answer about windows being laminated is wrong. Only the front windshield is laminated, and it doesn’t have that pattern.
That pattern is on tempered (safety) glass. This is because the process of tempering glass results in the outside of the glass being under considerable compressive stress. Coincidentally, this makes it both incredibly hard to break, but explode when that outside layer is broken.
The different levels of stress reflects light differently. That is what you are seeing. The pattern you see is the pattern of the machine used to manufacture the glass.
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