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Sci.chem FAQ - Part 5 of 7
Section - 20. Optical properties of chemicals

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Top Document: Sci.chem FAQ - Part 5 of 7
Previous Document: 19. Physical properties of chemicals
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20.1  Refractive Index properties and terminology 

When light passes between media of different density, the direction of the
beam is changed as it passes through the surface, and this is called
refraction. In the first medium, the angle between the light ray and the
perpendicular is called the angle of incidence (i), and the corresponding 
angle in the second medium is called the angle of refraction (r). The
ratio sine i / sine r is called the index of refraction, and usually the
assumption is that the light is travelling from the less dense (air) to more
dense, giving an index of refraction that is greater than 1. Although the
theoretical reference is a vacuum, air ( 0.03% different ) is usually used.
The refractive index of a compound decreases with increasing wavelength
( dispersion ), except where absorption occurs, thus the wavelength should 
be reported. The D lines of sodium are commonly used. 

The refractive index of a liquid varies with temperature and pressure, but 
the specific refraction ( Lorentz and Lorentz equation ) does not. The molar 
refraction is the specific refraction multiplied by the molecular weight,
and is approximately an additive property of the groups or elements 
comprising the compound. Tables of atomic refractions are available in the
literature, as are descriptions of the common types of refractometers [1].
 
20.2  Polarimetry properties and terminology 

Supplied by: Vince Hamner <vinny@vt.edu>

     Polarimetry is a method of chemical analysis that is concerned
with the extent to which a beam of linearly polarised light is rotated
during its transmission through a medium containing an optically active
species.[2]  Helpful discussions regarding polarised light may be found
elsewhere.[3,4]  In general, a compound is optically active if it has
no plane of symmetry and is not superimposable on its mirror image.
Such compounds are referred to as being "chiral".  Sucrose, nicotine,
and the amino acids are only a few of these substances that exhibit
an optical rotary power.

     A simple polarimeter instrument would consist of:

     1).  a light source -- typically set to 589 nm (the sodium "D" line)
     2).  a primary fixed linear polarising lens (customarily called the
          "polariser")
     3).  a glass sample cell (in the form of a long tube)
     4).  a secondary linear polarising lens (customarily called the
          "analyser") and
     5).  a photodetector.[5]

     Biot is credited with the determination of the basic equation
of polarimetry.[6,7]  The specific rotation of a substance (at a given
wavelength and temperature) is equivalent to the observed rotation (in
degrees) divided by the path length of the sample cell (in decimeters)
multiplied by the concentration of the sample (for a pure liquid,
-density- replaces concentration).  Influences of temperature,
concentration, and wavelength must always be taken into consideration.
If necessary, it is possible to apply corrections for each of these
variables.[8]  A few early contributors to our understanding of optical
activity and polarimetry include:  Malus, Arago, Biot, Drude, Herschel,
Fresnel, and Pasteur.

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Top Document: Sci.chem FAQ - Part 5 of 7
Previous Document: 19. Physical properties of chemicals
Next Document: 21. Molecular and Structural Modelling

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Last Update March 27 2014 @ 02:12 PM