Polarimetry/Catalog of Methods in Biophysics

Polarimetry is a method that uses the polarization properties of samples. Typically, the twist of the plane of polarization of the polarized light after passing through the sample is measured. It is used to study optically anisotropic materials and to measure the concentration of optically active substances bending the plane of polarization. This phenomenon is caused by the different refractive index for left-handed and right-handed circularly polarized waves.

Optical activity usually occurs in substances with chiral molecules with a predominance of one of the spatial conformations or in anisotropic crystalline substances.

The polarimeter consists of a pair of polarizers on a common optical axis, which can be rotated relative to each other, and the angle of mutual rotation is read on a scale. If the polarizers are crossed and thus their optical planes are rotated by 90°, light does not pass through. If the planes are coincident, light passes through at maximum intensity. If we insert a polarizing tube filled with an optically active substance between the polarizers, the light transmittance changes to a different value. During the subsequent rotation of one polarizer, the transmittance is minimized and the angle of mutual rotation of the polarizers is measured, which is a measure of the optical activity of the measured substance. A polarimeter in conjunction with a monochromator is used to measure optical circular dispersion ( ORD ) spectra of optically active substances.

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