Could someone 'splain this phenomenon? I place two polarized filters (A and B) at right angles to each other so that the first filter passes light that's polarized vertically, while the second passes light that's polarized horizontally. No light gets all the way through. OK so far. I now place a third polarized filter (C) between the first two. This one I angle so that it's diagonal (45 degrees from both A and B). Suddenly, some light gets through! Obviously, the act of polarizing the light as it passes through the first filter is having an effect on its subsequent polarization. Can someone explain what is happening? I read somewhere that this is a clear demonstration of QM. Is there a classical explanation?