Plane Polarisation

Light is a transverse wave: the oscillations (of the electric and magnetic fields) are perpendicular to the direction in which the wave travels and transfers energy.In unpolarised light, such as light from the Sun, a filament lamp or a candle, the oscillations occur in many different planes, all perpendicular to the direction of travel.In plane polarised light the oscillations are confined to one plane only, a plane that contains the direction of travel of the wave. For light, the plane of polarisation is usually described by the direction of the electric field oscillation.Only transverse waves can be polarised. In a longitudinal wave, such as sound, the oscillations are parallel to the direction of travel, so there is only one possible direction of oscillation and nothing to restrict. The fact that light can be polarised is therefore evidence that light is a transverse wave.All electromagnetic waves can be polarised. Many radio and television transmitters emit plane polarised waves, because the electrons in the transmitting aerial oscillate along its length. For the best reception, the receiving aerial must be lined up in the same plane as the transmitting aerial. The microwaves from a school microwave transmitter are also plane polarised.Mechanical transverse waves can be polarised too: a wave on a rope passed through a vertical slit keeps only its vertical oscillations.