Optical rotation or
optical activity (sometimes referred to as
rotary polarization) is the rotation of the plane of
polarization of linearly polarized light as it travels through certain materials. Optical activity occurs only in
chiral materials, those lacking microscopic mirror symmetry. Unlike other sources of
birefringence which alter a beam's state of polarization, optical activity can be observed in
fluids. This can include gases or solutions of
chiral molecules such as sugars, molecules with helical
secondary structure such as some proteins, and also chiral liquid crystals. It can also be observed in chiral solids such as certain crystals with a rotation between adacent
crystal planes (such as
quartz) or metamaterials. Rotation of light's plane of polarization may also occur through the
Faraday effect which involves a static magnetic field, however this is a distinct phenomenon that is not usually classified under "optical activity."