>>11356619No, malus's law explains the difference just fine. In the two-polarizer system, the second polarizer receives light polarized at 90 degrees, and cos^2(pi/2)=0 proportion of the light will get through.
With three polarizers, the second polarizer receives light at a non-90 degree angle, meaning some light gets through. This light is now polarized at the angle of the second polarizer, so a nonzero amount gets through the third polarizer.
The key here is that the light interacts with the polarizers, which changes the state of the light. There's nothing particularly revolutionary there. With single photons, the cos^2 becomes a probability of transmittance, rather than a proportion of total light transmitted.
The interesting results come when you start sending entangled photons through separate polarizers. Classically, if one of the pair interacts with a polarizer, the polarization state of the other photon shouldn't be affected. Yet experimental results indicate otherwise.
https://www.youtube.com/watch?v=sAXxSKifgtUhttps://www.youtube.com/watch?v=8UxYKN1q5sI 
