Polarimetry and Polarimeters – a simple explanation - Bellingham + Stanley - #1

Light travels at different speeds through different media and when a ray of light crosses the interface between two substances it changes direction. The emerging ray is called the refracted ray and the phenomenon is called refraction

The refractive index (Rl) of a substance, usually given the symbol n, is a measure of the speed of light through the substance and is defined as the ratio of the speed of light in the substance to the speed of fight In a vacuum. For practical purposes the speed of light in air rather than vacuum is used, the difference being very small.

Refractive Index of given substance (n) = Speed of light in substance

Speed of light in vacuum

The speed of light through a medium depends on the wavelength (or colour) of the light. Rl must therefore be defined at a specific wavelength, usually sodium light. For example, n_D denotes a refractive index based on the sodium D-line wavelength of 589 nm.

Rt is also a function of temperature. An increase in temperature usually gives rise to a decrease in density and light travels more rapidly through a lower density medium. Rl therefore tends to decrease with increasing temperature.

A refractometer is used to measure the refractive index of substances, usually liquids. Most refractometers are based on the critical angle effect, which defines the point of balance, the shadow point or borderline, ^CB

between refraction and total internal reflection of light at a prism/sample interface.

The Rl of the sample is derived from the geometry of the optical path and the refractive index of the prism material.

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