The orientation of light waves into a predominating angle. If all the waves in a beam of light move up and down or from side to side together, the beam is said to be plane-polarised. Some materials have the ability to filter out all light waves except those in a particular direction. Other materials are said to be optically active if they are able to change the angle of a plane-polarised beam of light. Liquid crystals often have this property and this enables them to be detected.