I have bought two cheap polarizing lenses for camera: New View Filtro Polarizzatore Circolare CPL Filter 58mm per Camera (Amazon Italia)
I have done the following observations:
If I put them one after the other in front of a common lamp and rotate them in any possible relative orientation, I NEVER get complete darkness,but I only get a mild loss of intensity and change of light color. I used to think that when polarization directions are orthogonal I should get complete darkness, why it is not so?
On the other hand, if I put one of them in front of a laptop display I easily find an orientation giving complete blackness.
I am in trouble in conciling the two "experiments": I can assume that the filters are no 100% efficient, but then why do I get complete blackness when filtering the polarized light coming from laptop display?
I suspect that those filters do not simply polarize at a given direction, but even thinking about circular polarization I cannot explain the dircrepancy.