Editing Optical rotation (section)

{{Short description|Rotation of the plane of linearly polarized light as it travels through a chiral material}}
{{Distinguish|circularly polarized light}}
[[File:Polarimeter (Optical rotation).svg|upright=1.5|thumb| Operating principle of a [[polarimeter]] for measuring optical rotation.
{{ordered list|
|Light source
|Unpolarized light
|[[Linear polarizer]]
|Linearly polarized light
|Sample tube containing molecules under study
|Optical rotation due to molecules
|Rotatable [[Polarimeter|linear analyzer]]
|Detector
}}]]

'''Optical rotation''', also known as '''polarization rotation''' or '''circular birefringence''', is the rotation of the orientation of the plane of [[polarization (waves)|polarization]] about the optical axis of [[linear polarization|linearly polarized]] light as it travels through certain materials. Circular birefringence and [[circular dichroism]] are the manifestations of '''optical activity'''. 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 [[fluid]]s. This can include gases or solutions of [[chirality (chemistry)|chiral molecules]] such as sugars, molecules with helical [[secondary structure]] such as some proteins, and also [[Liquid crystal#Chiral phases or twisted nematics|chiral liquid crystals]].  It can also be observed in chiral solids such as certain crystals with a rotation between adjacent [[crystal]] planes (such as [[quartz]]) or [[Metamaterial#Chiral|metamaterial]]s.

When looking at the source of light, the rotation of the plane of polarization may be either to the right ('''dextrorotatory''' or '''dextrorotary''' — ''d''-rotary, represented by (+), clockwise), or to the left ('''levorotatory''' or '''levorotary''' — ''l''-rotary, represented by (−), counter-clockwise) depending on which [[stereoisomer]] is 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.

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 [[enantiomer|enantiomeric ratio]] of chiral molecules in solution. Modulation of a liquid crystal's optical activity, viewed between two sheet [[polarizer#Absorptive polarizers|polarizers]], is the principle of operation of [[liquid-crystal display]]s (used in most modern televisions and computer monitors).