An ordinary transverse light wave has its electric-field vibrations spread over every direction in the plane perpendicular to the direction it travels; this is called unpolarised light. If those vibrations are restricted so they occur only along a single fixed direction in that perpendicular plane, the light is called plane polarised (or simply polarised) light; if the vibration amplitude varies with direction (largest along one axis, non-zero but smaller along the perpendicular axis) rather than being restricted entirely to one direction, the light is called partially polarised. The plane containing the actual field vibrations is the plane of vibration, and the plane perpendicular to it (containing the ray direction) is the plane of polarisation. Polarisation is the general phenomenon of restricting a transverse wave's vibrations to one particular direction; since interference and diffraction occur for both transverse and longitudinal waves, but polarisation is only possible for transverse waves, the existence of polarisation in light is the definitive proof that light waves are transverse, not longitudinal. Unpolarised light can be converted into plane polarised light by four distinct techniques: selective absorption (dichroic materials such as polaroid sheets, which transmit vibrations along one axis and absorb the rest), reflection (at the Brewster/polarising angle), double refraction (in crystals such as calcite, via a Nicol prism) and scattering (by small particles such as air molecules). In selective absorption, the first polaroid used to produce plane polarised light from an unpolarised beam is called the polariser, and a second polaroid used afterward to te …