Optical rotation

Unlike other sources of birefringence which alter a beam's state of polarization, optical activity can be observed in fluids.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).Dextrorotation and laevorotation (also spelled levorotation)[1][2] in chemistry and physics are the optical rotation of plane-polarized light.If a chiral molecule is dextrorotary, its enantiomer (geometric mirror image) will be laevorotary, and vice versa.For example, nine of the nineteen L-amino acids naturally occurring in proteins are, despite the L- prefix, actually dextrorotary (at a wavelength of 589 nm), and D-fructose is sometimes called "levulose" because it is levorotary.[6] The rotation of the orientation of linearly polarized light was first observed in 1811 in quartz by French physicist François Arago.Herschel discovered that different individual quartz crystals, whose crystalline structures are mirror images of each other (see illustration), rotate linear polarization by equal amounts but in opposite directions.[8] Jean Baptiste Biot also observed the rotation of the axis of polarization in certain liquids[9] and vapors of organic substances such as turpentine.[10] In 1822, Augustin-Jean Fresnel found that optical rotation could be explained as a species of birefringence: whereas previously known cases of birefringence were due to the different speeds of light polarized in two perpendicular planes, optical rotation was due to the different speeds of right-hand and left-hand circularly polarized light.In a similar manner, levulose, more commonly known as fructose, causes the plane of polarization to rotate to the left.[12] A solution of this compound derived from living things (to be specific, wine lees) rotates the plane of polarization of light passing through it, but tartaric acid derived by chemical synthesis has no such effect, even though its reactions are identical and its elemental composition is the same.In 1874, Jacobus Henricus van 't Hoff[13] and Joseph Achille Le Bel[14] independently proposed that this phenomenon of optical activity in carbon compounds could be explained by assuming that the 4 saturated chemical bonds between carbon atoms and their neighbors are directed towards the corners of a regular tetrahedron.[15] In 1898, Jagadish Chandra Bose described the ability of twisted artificial structures to rotate the polarization of microwaves.[16] In 1914, Karl F. Lindman showed the same effect for an artificial composite consisting of randomly-dispersed left- or right-handed wire helices in cotton.Extrinsic chirality associated with oblique illumination of metasurfaces lacking two-fold rotational symmetry has been observed to lead to large linear optical activity in transmission[24] and reflection,[25] as well as nonlinear optical activity exceeding that of lithium iodate by 30 million times.However, in 1988, M. P. Silverman discovered that polarization rotation can also occur for light reflected from chiral substances.[32] Shortly after, it was observed that chiral media can also reflect left-handed and right-handed circularly polarized waves with different efficiencies.If two enantiomers are present in equal proportions, then their effects cancel out and no optical activity is observed; this is termed a racemic mixture.At the fundamental level, polarization rotation in an optically active medium is caused by circular birefringence, and can best be understood in that way.Birefringence of this sort is possible even in a fluid because the handedness of the helices is not dependent on their orientation: even when the direction of one helix is reversed, it still appears right handed.And circularly polarized light itself is chiral: as the wave proceeds in one direction the electric (and magnetic) fields composing it are rotating clockwise (or counterclockwise for the opposite circular polarization), tracing out a right (or left) handed screw pattern in space.In addition to the bulk refractive index which substantially lowers the phase velocity of light in any dielectric (transparent) material compared to the speed of light (in vacuum), there is an additional interaction between the chirality of the wave and the chirality of the molecules.Unlike linear birefringence, however, natural optical rotation (in the absence of a magnetic field) cannot be explained in terms of a local material permittivity tensor (i.e., a charge response that only depends on the local electric field vector), as symmetry considerations forbid this.Now let us assume transmission through an optically active material which induces an additional phase difference between the right and left circularly polarized waves ofThe resulting variation in rotation with the wavelength of the light is called optical rotatory dispersion (ORD).So we find that the degree of rotation depends on the color of the light (the yellow sodium D line near 589 nm wavelength is commonly used for measurements) and is directly proportional to the path lengthAlthough optical activity is normally thought of as a property of fluids, particularly aqueous solutions, it has also been observed in crystals such as quartz (SiO2).This usage makes a polarimeter a tool of great importance to those trading in or using sugar syrups in bulk.Rotation of light's plane of polarization may also occur through the Faraday effect, which involves a static magnetic field.In case of optically active isotropic media, the rotation is the same for any direction of wave propagation.