Polarised light is quite simply light in which the waves are all vibrating in one fixed direction. Most waves (sound waves are an exception) involve a vibration at right angles to their path. Waves on water go only up and down but the waves on a wiggled rope can be made to go up and down or from side to side or in any other direction around their line of travel. In just the same way, light waves can vibrate in any direction across their path. Now in ‘ordinary’ unpolarised light the direction of vibration is fluctuating rapidly, on a time scale of about 10−8 s (a hundredth of a millionth of a second), and randomly through all possible directions around the path of the ray. Polarisation simply consists of forcing the waves to vibrate in a single, constant direction. A number of simple methods for showing that light is polarised and determining the direction of vibration will be described in this book, especially in chapters 2, 3 and 7.