I've been playing with 3D glasses. I punched out one of the filters, stacked them with some space in between and looked through both the filters without flipping anything over. I expected the interaction to be like between linear polarization filters at 90° angle - no light should pass. If they were linear polarization filters at 90° all light that passed throught the first filter would be polarized according to the first filter and would not be able to pass through the second. So I thought all light that was able to pass through the first circular polarization filter would have, lets say clockwise polarization and it will not be able to pass through the second filter, because the second would only let counter-clockwise polarized light through. But actually a lot of light passes through both the filters - it only changes its tint between blue and yellow depending on the angle I twist them at.
I know the equation describing the chance of a linearly polarized photon to pass through a linear filter, but I don't know any such equations about circular filters.