One problem in my hw assignment that I am having trouble with is: Take two ideal polarizers (the first with its axis vertical and the second, horizontal) and insert between them a stack of 10 half-wave plates, the first with its fast axis rotated pi/20 rad. from the vertical, and each subsequent one rotated pi/20 rad. from the previous one. Determine the ratio of intensity after the second polarizer to that after the first polarizer. Does anyone know how to solve this problem?