Technical Bulletin - Principles of Refractometry0 pages

Bellingham + Stanley
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TECHNICAL BULLETIN - R001
Title:

Principles of Refractometry

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.

Release date: 2015

©Xylem Inc. 2015