Optical activity is the property, possessed by certain substances, of rotating the plane of polarization of polarized light passing through them. This optical rotation is measured using a polarimeter.

 

Optical activity is shown by asymmetric crystals which have two mirror-image forms – the rotation being to the left (levorotatory, l-) or right (dextrorotatory, d-) respectively – and by compounds with asymmetric molecules showing optical stereoisomers.

 

Magnetic rotation of the plane of polarization is known as Faraday rotation.