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Malus's law gives the intensity through a second polariser as I0 cos^2(theta). At 30 degrees, 75% gets through; at 45 degrees exactly half; at 90 degrees none at all. The cosine applies to amplitude and the square to intensity, which is why crossed polarisers go completely black.
Malus's law
I = I0 cos^2(theta)
Amplitude
amplitude ratio = cos(theta)
Because cos^2(45) is 0.5. That is also why inserting a third polariser at 45 degrees between crossed ones lets light through again.
No. Unpolarised light loses half its intensity at the first polariser regardless of angle; Malus's law applies from the second onwards.