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The well known Malus law predicts $\cos^2\theta$ for the probability of passing through a filter oriented with an angle $\theta$ w.r.t. the polarization direction of the incident photon. On the other hand the standard quantum treatment of spin $1/2$ particles gives for the same question the result $\cos^2\frac{\theta}{2}=\frac{1+\cos\theta}{2}$ (see for example http://www.lecture-notes.co.uk/susskind/quantum-entanglements/lecture-4/measurement/ ). Is this difference coming only fron the spin (1 vs. $1/2$) or am I comparing apples and oranges? My question is motivated by the fact that in many treatments of EPR experiments people speak of electron and photons while using always the simple Malus law.

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  • $\begingroup$ The fact that the photon is massless turns out to be very important in this regard. $\endgroup$ – dmckee Sep 19 '17 at 17:54
  • $\begingroup$ Coulld you elaborate on this? The fact that the photon is massless implies that only two states are available, so in principle this could be assimilated to spin $1/2$, but I am unsure about the details. $\endgroup$ – Arnaldo Maccarone Sep 20 '17 at 6:53
  • $\begingroup$ I believe your question is answered here $\endgroup$ – Yuval Nissan Mar 11 '18 at 16:11

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