About refractometry – Nova-Tech iPR-Series User Manual
Page 7
Prism with
sample
Light source
CCD-diode
array
Total reflected light
0° 15° 30° 45° 60°
0°
20°
42°
70°
Air
90°
Refracted
beam
Water
_
G
Critical angle
Luft
Water
Total reflexion
Total
reflected
light
In case the incident angle is 60° there will be no refracted
ray any more, we would say: light is totally reflected.
A more detailed picture below shows the case where
light impinges under such special angle that the refracted
beam makes an angle of 90° with the surface normal.
There comes a time when all of the rays are reflected, this
happens when the angle of incidence is equal to or greater
than the critical angle
_
crit
.
If one knows exactly the refractive index of the glass prism
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measuring the critical angle of total reflection, one could
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Since the light beam only probes a fraction of wavelength
into the second medium very dark and turbid samples can
be measured without problems. By inserting a drop of
liquid on top of the measuring prism a very sharp line will
appear dividing regions of below and above the critical
angle.
Temperature effects
A solution of 40 g of sucrose in 100 g of water has a
refractive index (RI) of 1.39986 at a temperature of 20°C.
The same solution has a much lower RI value 1.39828 at
30°C.
The difference in the measured values is caused only by
the change of the temperature and not by change of
the concentration. The so called temperature correction
therefore considers the influence of the temperature on
the solution to be measured. This is generally a non-linear
behaviour (matrix) in dependency of the different concen-
trations.
Herewith it is possible to dertermine a temperature cor-
rection for a substance and to program a refractometer
in a way that it indicates only the concentration of this
substance independent of the measuring temperature.
About Refractometry
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In a vacuum, light travels at a maximum speed of about
300.000 km/s while travelling through water, the speed is
about 225.000 km/s, which is 25% less. In a sapphire it
will only reach 170.000 km/s.
A refractometer is a measuring instrument for the speed
of light. The result will not be indicated directly but related
to the speed of light in air. This comparison is called
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.
The indication, that a certain material has a refractive index
of 1.5 thus means, that the speed of light travels 50%
faster through air than through this material.
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It depends on temperature and wavelength (
h = colour)
of the light. Thus using a refractometer, will enable you
to determine the concentration of a material, if tempe-
rature and wavelength are known. But it is also possible,
that different materials have the same refractive index
at various concentrations. Thus a clear determination of
liquid substances may only be successful with binary mix-
tures (Mixtures consisting of two compounds).
In practice, the refractive index determines the mixing
ratio also of multicompound solutions quite exactly and
easily as in general only the concentration of one of the
componends needs to be determined. Thus it is a quanti-
tative measurement.
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and the composition of many two-compound solutions.
The best known example for such a mixture is a solution
of sucrose in water, which has been studied throughly. A
refractometer can be grated in a way that the value may
be indicated directly as dry substance %RTS. For sucrose,
this unit is also named #SJY (abb. Bx).
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To understand total internal reflection, we begin with a
thought experiment.
Suppose that a laser beam inside of a water tank aims
towards the air-water boundary. Then suppose that the
angle at which the beam directed upwards is slowly altered,
beginning with small angles of incidence and proceeding
towards larger and larger angles of incidence.
The principles of boundary behaviour let us expect that we
would observe both reflection and refraction. We would
also observe that the intensity of the reflected and refrac-
ted rays do not remain constant. At angle of incidence
close to 0 degrees, most of the light energy is transmitted
across the boundary and very little of it is reflected. As the
angle is increased to greater and greater angles, we would
begin to observe less refraction and more reflection. That
is, as the angle of incidence is increased, the brightness
of the refracted ray decreases and the brightness of the
reflected ray increases.
Finally, we would observe that the angles of the reflection
and refraction are note equal. Since the light waves would
refract away from the normal, the angle of refraction
would be greater than the angle of incidence.