Interesting things start to happen when polarised light passes through cellophane. A simple jam-pot cover, obtainable in packets of 20 from a newsagent, can rotate the direction of polarisation by 90 ◦. One such film, placed between crossed polarisers, can twist the direction of vibration of light from the first polariser so that it then passes freely through the second. It thus appears as a clear, circular ‘window’ through the darkened background-the effect is especially striking when done on an overhead projector (figure 2.1). But this only happens with certain orientations of the disc, for turning it makes the ‘window’ darken and brighten four times during each rotation. The explanation for this depends on a property of the film called birefringence.