Cross two polarizers and all light is blocked. Inserting a third polarizer in between two crossed polarizers and light again reemerges from the polarizers. Why is this? I'm trying to wrap my mind around this. I noticed that certain transparent plastics (cellophane, e.g.,) also allow some light to pass through the crossed polarizers because the plastic acts as a sort of 1/2 wave plate. Could it be that the third polarizer is also acting as a 1/2 wave plate, allowing light to transmit through the crossed polarizers? Again, how can light be transmitted through crossed polarizers when a third polarizer is introduced in between the crossed polarizers?