Technical Bulletin - Principles of Refractometry
3Pages

Bellingham + Stanley Longfield Road, Tunbridge Wells, Kent, TN2 3EY, UK Tel +44 (0) 1892 500400 Fax +44 (0) 1892 543115 TECHNICAL BULLETIN - R001 Title: 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 (RI) 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 light 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 vacuum ____________________________________ Speed of light in substance The speed of light through a medium depends on the wavelength (or colour) of the light. RI must therefore be defined at a specific wavelength, usually sodium light. For example, nD denotes a refractive index based on the sodium D-line wavelength of 589 nm. RI 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. RI 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, between refraction and total internal reflection of light at a prism/sample interface. The RI of the sample is derived from the geometry of the optical path and the refractive index of the prism material. Every effort has been made to ensure the accuracy of the contents of this Bulletin. However, Bellingham + Stanley assumes no responsibility for errors contained herein or their consequences.

Bellingham + Stanley Longfield Road, Tunbridge Wells, Kent, TN2 3EY, UK Tel +44 (0) 1892 500400 Fax +44 (0) 1892 543115 Modern digital automatic refractometers use a light sensitive integrated circuit (self-scanned array) to detect the exact position of the borderline. Bellingham + Stanley instruments also incorporate special software for interpreting ‘fuzzy’ borderlines that many products tend to produce. In this way, there is no subjective human judgement needed in taking a reading - a major advantage of an automatic instrument. When the composition of a substance changes, so does its RI....

Bellingham + Stanley Longfield Road, Tunbridge Wells, Kent, TN2 3EY, UK Tel +44 (0) 1892 500400 Fax +44 (0) 1892 543115 Abbe Refractometers These are optical-mechanical instruments and they derive their name from the 19th Century physicist, Ernst Abbe. Automatic Digital Refractometers These are solid state electronic instruments incorporating flexible software and designed primarily for use in quality control applications. High accuracy may be achieved with both Abbe and Digital Automatic refractometers. The latter are generally more expensive instruments and tend to be favoured by quality...