Optical rotation
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In optics, optical rotation or optical activity is the rotation of linearly polarized light as it travels through certain materials. It occurs in solutions of chiral molecules (e.g. sugar), solids with rotated crystal planes (e.g. quartz), and spin-polarized gases of atoms or molecules. It is used in sugar industry to measure syrup concentration, in optics to manipulate polarization, in chemistry to characterize substances in solution, and is being developed as a method to measure blood sugar concentration in diabetic people.
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History
The rotation of the orientation of linearly polarized light was observed in the early 1800's (Jean Baptiste Biot was one of the early investigators) before the nature of molecules was understood. 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 glucose, dextrose, refers to the fact that it causes linearly polarized light to rotate to the right or dexter side. Similarly, 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, formed by adding fructose to a solution of glucose, gets its name from the fact that the conversion causes the direction of rotation to "invert" from right to left.
Theory
Optical activity is a type of birefringence. Any linear polarization of light can written as an equal combination of right-hand (RHC) and left-hand circularly (LHC) polarized light:
- <math>\mathbf{E}_{\theta_0}=\mathbf{E}_{RHC}+e^{i2\theta_0}\mathbf{E}_{LHC},</math>
where <math>\mathbf{E}</math> is the electric field of the light. The relative phase between the two circular polarizations, <math>2\theta_0</math>, sets the direction of the linear polarization to <math>\theta_0</math>. In an optically active material the two circular polarizations experience different refractive indices. The difference in the indices quantifies the strength of the optical activity,
- <math>\Delta n=n_{RHC}-n_{LHC}</math>.
This difference is a characteristic of the material (for substances in solution it is given as the specific rotation). After travelling through length <math>L</math> of material the two polarizations pick up a relative phase of
- <math>2\Delta \theta=\frac{\Delta n L2\pi}{\lambda}</math>,
where <math>\lambda</math> is the wavelength of the light (in vacuum). Consequently, the final polarization is rotated to angle <math>\theta_0+\Delta \theta</math>.
Generally, the refractive index depends on the wavelength (see dispersion). The variation in rotation with the wavelength of the light is called optical rotatory dispersion (ORD). ORD spectra and circular dichroism spectra are related through the Kramers-Kronig relations. Complete knowledge of one spectrum allows the calculation of the other.
In summary, 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), the path length <math>L</math> and the properties of the material (e.g. <math>\Delta n</math> or specific rotation and concentration).
Areas of use
For a pure substance in solution, if the color and path length are fixed and the specific rotation is known, the observed rotation can be used to calculate the concentration. This usage makes a polarimeter a tool of great importance to those who trade in or use sugar syrups in bulk.
In the presence of magnetic fields all molecules have optical activity. A magnetic field aligned in the direction of light propagating through a material will cause the rotation of the plane of linear polarization. This Faraday effect is one of the first discoveries of the relationship between light and electromagnetic effects.
Optical activity or rotation should not be confused with circularly polarized light. Circularly polarized light is often presented as a linearly polarization rotating as the light propagates. However, in this picture the polarization completely rotates in a length equal to the wavelength (roughly one micrometre) and it can happen in vacuum. In contrast, optical activity only occurs in a material and a complete rotation occurs in a length of millimeters to meters, depending on the material.
See also
de:Optische Aktivität fr:Pouvoir rotatoire pl:Aktywność optyczna