how is the QM prediction derived for Bell's tests?

Question 1: how is the QM prediction (i.e. cosine curve, shown below) derived for Bell's tests? in layman' termss

Question 2: while Malus's law, some physicists say, is not related to this cosine curve.
Is there an indirect/lengthy way to connect the two?

 Recognitions: Gold Member Science Advisor Great question! I have been meaning to prepare a full explanation of that, but of course have not yet. However, you can get a hint from this: http://departments.colgate.edu/physi...nglement09.PDF See (8) and (9) for the theta=0, and (12) and (13) for the case for general theta. Basically, as you switch to a rotated basis, the cos factors start slipping in.