আলোক আবর্তন

আলোক আবর্তন (Optical rotation বা পোলারাইজেশন রোটেশন বা সারকুলার বাইরেফ্রিজেন্স) হল নির্দিষ্ট পদার্থের মধ্য দিয়ে যাতায়াত করার সময় রৈখিক সমবর্তন আলোকের অপটিক্যাল অক্ষ সম্পর্কে সমবর্তন এর তলের অভিমুখীকরণের আবর্তন। সারকুলার বাইরেফ্রিজেন্স এবং সারকুলার ডিসক্রোইজম হ'ল অপটিকাল ক্রিয়াকলাপ এর প্রকাশ। অপটিকাল ক্রিয়াকলাপ কেবল কাইরাল পদার্থে ঘটে থাকে যাদের অণুবীক্ষণিক আয়নার প্রতিসাম্যের অভাব রয়েছে।  অপটিকাল কার্যকলাপ তরল এ লক্ষ্য করা যায়। এর মধ্যে কাইরাল অণু যেমন শর্করা, হেলিকাল গৌণ কাঠামো সহ কিছু প্রোটিনযুক্ত অণু এবং কাইরাল তরল স্ফটিক এর গ্যাস বা দ্অরবণন্তর্ভুক্ত থাকতে পারে। এটি কাইরাল কঠিন যেমন নির্দিষ্ট স্ফটিকেও সংলগ্ন স্ফটিক তলের (যেমন কোয়ার্টজ) অথবা উপাদান এর মধ্যে লক্ষ্য করা যায়।

The rotation of the plane of polarization may be either clockwise, to the right (dextrorotary — d-rotary,represented by (+), or to the left (levorotary — l-rotary ,represented by (-) 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.

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).

ফ্যারাডে এফেক্টের সাথে তুলনা 
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 classified under "optical activity." Optical activity is reciprocal, i.e. it is the same for opposite directions of wave propagation through an optically active medium, for example clockwise polarization rotation from the point of view of an observer. In case of optically active isotropic media, the rotation is the same for any direction of wave propagation. In contrast, the Faraday effect is non-reciprocal, i.e opposite directions of wave propagation through a Faraday medium will result in clockwise and anti-clockwise polarization rotation from the point of view of an observer. Faraday rotation depends on the propagation direction relative to that of the applied magnetic field. All compounds can exhibit polarization rotation in the presence of an applied magnetic field, provided that (a component of) the magnetic field is oriented in the direction of light propagation. The Faraday effect is one of the first discoveries of the relationship between light and electromagnetic effects.

ইতিহাস 

The rotation of the orientation of linearly polarized light was first observed in 1811 in quartz by French physicist François Jean Dominique Arago. In 1820, the English astronomer Sir John F.W. 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. Jean Baptiste Biot also observed the rotation of the axis of polarization in certain liquids and vapors of organic substances such as turpentine. Simple polarimeters have been used since this time to measure the concentrations of simple sugars, such as glucose, in solution. In fact one name for D-glucose (the biological isomer), is dextrose, referring to the fact that it causes linearly polarized light to rotate to the right or dexter side. In a similar manner, levulose, more commonly known as fructose, causes the plane of polarization to rotate to the left. Fructose is even more strongly levorotatory than glucose is dextrorotatory. Invert sugar syrup, commercially formed by the hydrolysis of sucrose syrup to a mixture of the component simple sugars, fructose, and glucose, gets its name from the fact that the conversion causes the direction of rotation to "invert" from right to left.

In 1849, Louis Pasteur resolved a problem concerning the nature of tartaric acid. 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. Pasteur noticed that the crystals come in two asymmetric forms that are mirror images of one another. Sorting the crystals by hand gave two forms of the compound: Solutions of one form rotate polarized light clockwise, while the other form rotate light counterclockwise. An equal mix of the two has no polarizing effect on light. Pasteur deduced that the molecule in question is asymmetric and could exist in two different forms that resemble one another as would left- and right-hand gloves, and that the organic form of the compound consists of purely the one type.

In 1874, Jacobus Henricus van 't Hoff and Joseph Achille Le Bel 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. If the 4 neighbors are all different, then there are two possible orderings of the neighbors around the tetrahedron, which will be mirror images of each other. This led to a better understanding of the three-dimensional nature of molecules.

In 1945, Charles William Bunn predicted optical activity of achiral structures, if the wave's propgation direction and the achiral structure form an experimental arrangement that is different from its mirror image. Such optical activity due to extrinsic chirality was observed in the 1960s in liquid crystals.

In 1950, Sergey Vavilov predicted optical activity that depends on the intensity of light and the effect of nonlinear optical activity was observed in 1979 in lithium iodate crystals.

Optical activity is normally observed for transmitted light, however, in 1988, M. P. Silverman discovered that polarization rotation can also occur for light reflected from chiral substances. Shortly after, it was observed chiral media can also reflect left-handed and right-handed circularly polarized waves with different efficiencies. These phenomena of specular circular birefringence and specular circular dichroism are jointly known as specular optical activity. Specular optical activity is very weak in natural materials.

In 1898 Jagadish Chandra Bose described the ability of twisted artificial structures to rotate the polarization of microwaves. Since the early 21st century, the development of artificial materials has led to the prediction and realization of chiral metamaterials with optical activity exceeding that of natural media by orders of magnitude in the optical part of the spectrum. 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 and reflection, as well as nonlinear optical acitvity exceeding that of lithium iodate by 30 million times.

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