La rotación óptica o actividad óptica es la rotación de la polarización lineal de la luz cuando viaja a través de ciertos materiales. Suele ser un fenómeno que ocurre en soluciones que presentan moléculas quirales tales como la sacarosa (azúcar), sólidos con planos cristalinos rotados, tales como el cuarzo, y la polarización circular de gases atómicos o moleculares. Se emplea en la industria de elaboración de azúcar para medir en los siropes la concentración de azúcares, en óptica para manipular la polarización, en química para caracterizar sustancias en solución acuosa, y en medicina está siendo evaluado en la actualidad como un método de determinación de la concentración de azúcar en sangre en casos de personas que sufren la diabetes.

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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." The rotation of the plane of polarization may be either clockwise, to the right (dextrorotary — d-rotary), or left (levorotary — l-rotary) depending on which stereoisomer is present (or dominant). For instance, sucrose and camphor are d-rotary whereas cholesterol is l-rotary. For a given substance, the angle by which the polarization of light of a specified wavelength is rotated is proportional to the path length through the material and (for a solution) proportional to its concentration. The rotation is not dependent on the direction of propagation, unlike the Faraday effect where the rotation is dependent on the relative direction of the applied magnetic field. Optical activity is measured using a polarized source and polarimeter. This is a tool particularly used in the sugar industry to measure the sugar concentration of syrup, and generally in chemistry to measure the concentration or enantiomeric ratio of chiral molecules in solution. Modulation of a liquid crystal's optical activity, viewed between two sheet polarizers, is the principle of operation of liquid-crystal displays (used in most modern televisions and computer monitors).

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