Polarimetry

Polarimetry is the measurement of the polarisation of light; a polarimeter is the scientific instrument used to make these measurements. Polarimetry of thin films and surfaces is commonly known as ellipsometry.

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Polarimetry can be used to measure various optical properties of a material, including linear birefringence, circular birefringence (also known as optical rotation or optical rotary dispersion), linear dichroism, circular dichroism and scattering.

To measure these various properties, there have been many designs of polarimeters. Some are archaic and some are in current use. The most sensitive polarimeters are based on interferometers, while more conventional polarimeters are based on arrangements of polarising filters, wave plates or other devices.

Measuring optical rotation

Optically active samples, such as solutions of chiral molecules, often exhibit circular birefringence. Circular birefringence causes rotation of the polarisation of plane polarised light as it passes through the sample.

A simple polarimeter to measure this rotation consists of a long tube with flat glass ends, into which the sample is placed. At each end of the tube is a Nicol prism or other polarizer. Light is shone through the tube, and the prism at the other end, attached to an eye-piece, is rotated until all light is shut off. The angle of rotation is then read off of a scale. The specific rotation of the sample may then be calculated. Temperature can affect the rotation of light which should be accounted for in the calculations.