I read about one version of the double-slit experiment that uses a light beam from a laser and a combination of fully-silvered and half-silvered mirrors. The author, Roger Penrose, describes one part of the experimental apparatus in which a light beam from a laser, traveling “north”, encounters a fully-silvered mirror at a 45 degree angle to the beam and is reflected “east”. He said that the wave function for those photons from the laser had to be multiplied by i in order to account for the ¼ phase shift (pi/2) caused by the mirror. My question is this: Is the pi/2 phase shift caused by the fact that the laser beam changed direction by 90 degrees or is the phase shift due solely to the physical properties of the mirror? Thanks in advance.