It isn't absolutely necessary to use two polarizers, if you have a pair of polarizing sunglasses.? Indirect light from the sky on a clear day is partially polarized, so just put them on and look at the sky through the glass.? Sky light isn't 100% polarized so the contrast between the light/dark regions won't be as dark as with a pair of polarizing filters.? But just about everyone has some polarizing sunglasses lying around.
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I see this effect most often when driving since the car's windows are made with tempered glass.
