Most light consists of light waves vibrating randomly - up and down, left to right or something in between. A single light wave (or photon) actually consists of a vibrating electrical component and, at right angles to this, a vibrating magnetic component. We can pass light through a filter, called a polarising filter, which makes the electric component vibrate in a fixed plane. Light for which the electrical component is made to vibrate in fixed direction is said to be plane polarised. The magnetic component is still there, vibrating perpendicular to the electric component.