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Refractometry
Introduction to the Theory of Refractometry
Refraction of light is caused by different propagation of speed of light in different media. If light traverses
the interface between dissimilar materials, it will change its propagation direction depending on the speed
difference. This phenomenon is called " refraction". This gives to the whole subject the name "Refractome-
try", standing for all methods including refractive index determination for controlling substance properties.
Refractive index 12 for light changeover from to equals exactly to the ratio of speed of light in these two
media:
The refractive index is also dependent on the wavelength of light. The refraction of long-waved light is
higher than the refraction of short-waved light. This behavior is called "Dispersion". That is why a correct
declaration of refractive index requires also the wavelength. Normally refractive index is given for the yel-
low line of sodium (D-Line), which equates to a wavelength of 589.3 nm.
If light, coming from a high refracting material, is entering a lower refractive medium under a high angle
of incidence, then this will result in a striking escape of light into the second media. Further increase of
the incident angle results in a totally reflected beam where the surface behaves as a perfect mirror. If one
brings now a third medium onto this totally reflecting prism surface, then for certain incident angles the
total reflection will be disturbed and light can enter the third medium, which normally is a liquid. In this way
a sharply bounded line in the reflected light will be produced, whose position can be easily detected by a
CCD-sensor and the software of the instrument. An increase of the sample refractive index will result in a
movement of the borderline meaning one has a very sensitive sensor for refractive indices.
10
n
= c
/ c
12
1
2
c
= Speed of light in Medium 1
1
c
= Speed of light in Medium 2
2
Law of Refraction:
n
· sin(a
) = n
· sin(a
1
1
1
a = Angle of incidence
-continued-
)
2
1400700-101 Rev. A
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