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Optical rotation, also known as polarisation rotation or circular birefringence, is the rotation of the plane of polarisation about the optical axis of linearly polarised light when it travels through specific materials. Optical activity manifests as circular birefringence and circular dichroism. Only chiral materials, which lack microscopic mirror symmetry, exhibit optical activity. Unlike other sources of birefringence that change the state of polarisation of a beam, optical activity in fluids can be observed. Gases or solutions of chiral molecules such as sugars, molecules with a helical secondary structure such as some proteins, and chiral liquid crystals are examples of this.
It can also be seen in chiral solids like certain crystals with a rotation between adjacent crystal planes (like quartz) or metamaterials. Depending on which stereoisomer is dominant, the rotation of the plane of polarisation may be to the right (dextrorotary — d-rotary, represented by (+), clockwise) or to the left (levorotary — l-rotary, represented by (), counter-clockwise). Sucrose and camphor, for example, are d-rotary, whereas cholesterol is l-rotary. For a given substance, the angle at which the polarisation of light of a specific wavelength is rotated is proportional to the path length through the material and (for a solution) proportional to its concentration.
A polarised source and polarimeter are used to measure optical activity. This is a tool used primarily in the sugar industry to determine the sugar concentration of syrup, but it is also used in chemistry to determine the concentration or enantiomeric ratio of chiral molecules in solution. The principle of operation of liquid-crystal displays is the modulation of a liquid crystal’s optical activity when viewed between two sheet polarizers (used in most modern televisions and computer monitors).
Overview
When polarised light passes through a layer of liquid, the optical rotation is the angle at which the plane of polarization rotates. The ability of a compound to rotate the plane of polarised light is referred to as optical activity. This property results from the interaction of polarised light’s electromagnetic radiation with the unsymmetric electric fields generated by the electrons in a chiral molecule. When linearly polarised light is rotated while passing through a compound, it is said to be optically active. When polarised light passes through a layer of liquid, the optical rotation is the angle at which the plane of polarisation rotates. The effect of optical rotation in a substance is determined by the concentration of chiral molecules and their molecular structure. Every optically active substance has a unique rotation.
The optical rotation, or rotation of the plane of polarised light, is depicted below.
The tendency of a substance to rotate the plane of polarisation of plane-polarized light is referred to as optical activity. Optically active substances are those that have the ability to rotate the plane of polarised light passing through them. Optimally active crystals include quartz and cinnabar, while optically active solutions include sugar and tartaric acid aqueous solutions.
A polarimeter is a device used to measure optical rotation. The observed rotation has a linear relationship with the concentration of optically active compound in the sample. The observed rotation and the wavelength of light used have a nonlinear relationship. Concentrate on the features and limitations to fully comprehend them and build your conceptual foundation.
Optically active substance
The ability of a substance to rotate the plane of polarisation of a light beam passing through it is referred to as optical activity. (The vibrations of the electric field are confined to a single plane in plane-polarized light.) The intensity of optical activity is expressed in terms of a quantity known as specific rotation, which is defined by an equation that relates the angle of rotation of the plane, the length of the light path through the sample, and the density of the sample .
There are two types of optically active substances:
- Dextrorotatory Substances: Dextrorotatory substances are also referred to as right substances. Dextrorotatory Substances are right-handed or dextrorotatory optically active substances that rotate the plane of polarisation of light towards the right. In other words, Dextrorotatory substances are those that rotate plane-polarized light to the right or clockwise direction.
- Laevorotatory Substances: Laevorotatory Substances are left-handed or Levorotatory substances that rotate the plane of polarisation of light toward the left. Left-handed Levorotatory substances are optically active substances that rotate the plane of polarisation of light toward the left. In other words, Dextrorotatory substances are those that rotate plane-polarized light to the left or counterclockwise direction.
Optical activity examples
Compounds that rotate the plane of polarised light are referred to as optically active, and this property is referred to as optical.
Because they both have one chiral centre, they are optically active compounds.
- The optical rotation method is used to calculate the percentage of optically active substance in a solution.
- The sugar level in a diabetic’s urine is calculated by calculating the angle of rotation of the plane of polarisation.
- Optical activity is a time-dependent property that is used to calculate kinetic reactions.
- The optical rotation is also used to plot optical rotatory dispersion curves for various wavelength ranges, which aids in the analysis of molecular structure.
- At the wavelength specified in the monograph, the optical activity is measured on a layer of suitable thickness.
Specific optical rotation
The angle of rotation measured as defined in the monograph, calculated with reference to a layer 100mm thick, and divided by the relative density determined at the temperature at which the rotation is measured, is the specific optical rotation of a liquid substance. The rotation produced by a 1mm slab for a solid or a 100mm path length for a liquid is a measure of a sample’s optical activity. This is known as the specific rotation. Liquids typically rotate light much less than solids. Solid solutions will obviously exhibit an effect that is dependent on the concentration of active material and, to a lesser extent, on temperature and solvent.
The effect of optical rotation in a substance is determined by the concentration of chiral molecules and their molecular structure. Each optically active substance will have a unique rotation. The optical activity will either increase or decrease depending on the concentration level. Many students will have questions about what optical activity is in chemistry. Because the optical activity is found mostly in chiral substances, it is equally important in chemistry.
FAQs
What exactly is Specific Rotation?
Specific rotation is a property of a substance that serves as the standard measurement for optical rotation in that substance. When light passes through a sample, it changes the direction of plane-polarized light per unit distance of the cell and per unit concentration of the sample. Specific rotation is also an intrinsic property, which means that it is a property that a substance possesses independently of other factors.
What exactly is optical rotation?
When polarised light passes through a layer of liquid, the optical rotation is defined as the angle at which the plane of polarisation rotates. Optical activity is also defined as a compound's ability to rotate the plane of polarised light. The optical rotation property results from the interaction of polarised light's electromagnetic radiation with the unsymmetric electric fields generated by the electrons in a chiral molecule.
