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Refractometer @ Jewel Info 4 U
By:Ritika Changrani
Summary: A Refractometer
is a gemological tool used to find
the ability of gemstone to bend or
refract light. This measure is the
refractive index of a gemstone.
A Refractometer can be used to
measure the refractive index of both
singly refractive (isotropic) and
doubly refractive (anisotropic)
gems. The refractive index helps to
determine the composition and
physical properties of gemstone.
With the help of refractive index,
it is easy to identify the type of
gemstone. Also, it is possible to
estimate the amount of impurity
present in the gemstone by comparing
the measured refractive index with
the standard value of the natural
gemstone. With the help of
Refractometer, birefringence and
optic sign of the gem can also be
determined.
How does a Refractometer work?
 Refractometer works on the principle
of Total Internal Reflection i.e.
when light travels from denser
material or the material with higher
refractive index to rarer material
or the material with lower
refractive index, the light reflects
into the denser material or refracts
into the rarer material depending on
the angle of incidence of the light
traveling. (image courtesy:
www.diamondtech.com).
Refractometer is made of a metal
case, polarizing filter on the top,
glass hemicylinder and viewing lens
at the bottom. Natural or white
light is passed through the back of
the unit. The light travels through
the glass hemi cylinder and then to
the gemstone which is placed on the
glass hemicylinder using a special
refractive index liquid, which seals
the gemstone to the hemicylinder
thereby removing the air in between
them. This is done to maintain the
same speed of light in both the
glass hemicylinder as well as stone
so that the refracted rays obtained
are accurate. The light is then
refracted and sent to the viewing
lens through the glass hemicylinder.
Through the viewing lens one can see
the refracted rays of light. The
bending or slowing of light caused
by the gemstone appears as the light
and dark area combined by thin green
line. The green line thus obtained
is the refractive index of the
gemstone.
A Refractometer is designed such
that the incident and exiting rays
reach the boundary of hemicylinder
at 90 degree. No refraction occurs
when the light ray is at 90 degrees
to the boundary. Therefore no
refracted rays are visible and hence
appear as dark areas on the scale.
The reflected rays of light appear
as the light areas on the scale of
the Refractometer. The light and the
dark boundary on the scale is the
representation of critical angle.
How to use a Refractometer?
- Apply a small amount of refractive
index liquid on the metal base near
the hemicylinder. Slowly place the
gemstone on the refractive index
liquid. Then slide the stone to the
centre of the hemicylinder. Take
care that you don't apply pressure
on the stone as it may damage the
hemicylinder. The liquid builds
optical contact between the stone
and the hemicylinder.
- Close the lid of the Refractometer
to avoid external light.
- Place the light source at the back
of the Refractometer. Adjust the
beam until you see bright light when
you observe through the viewing
lens.
- Position your eye such that you
can see the whole scale through the
viewing lens without moving your
eye. This is to avoid the parallax
error in the reading.
- Move your head up and down until
you can see the clear separation of
light and dark areas. Note down the
reading on the scale where the thin
greenish line appears.
- Unscrew the polarizing filter and
place it on the viewing lens.
- Take down the reading. Rotate the
polarizing filter to 90 degree and
note the readings.
- Polarizing filter helps to
determine if the stone is singly
refractive or doubly refractive. If
the stone is singly refractive, only
one shadow edge is seen. Note down
the readings rotating the stone 135
degrees.
- If the stone is doubly refractive,
shadow edges move between two values
on the scale. Note down the two
values, i.e. the upper and the lower
value. Take down four sets of
reading by rotating the stone to 45
degrees.
- If one of the reading i.e. either
upper reading or the lower reading
remains same, the stone is said to
be uni-axial. If both the readings
change after rotating the stone, the
stone is said to be bi-axial.
- The refractive index is the
difference between the largest
higher reading and the smallest
lower reading.
- f the difference is positive, the
optic sign of the stone is positive;
else the optic sign of the stone is
negative.
Tips to Remember while using a
Refractometer
- Store the equipment in a dry place
to protect the device from moisture
which can affect the accuracy of the
equipment.
- While you move the gem stone on
the glass hemicylinder, take care to
move the stone slowly so that no
scratch is caused on the
hemicylinder. A minute scratch
causes can damage it and turns
costly to be repaired.
- The refractive index liquid has to
be stored and sealed. Avoid its
contact with hands or eyes.
- Make sure to use the tiniest drop
of refractive index liquid since the
liquid itself has its own refractive
index and causes total internal
reflection between hemicylinder and
the liquid. This will in turn affect
the accuracy of the reading.
- The contact liquid should be
selected with proper care since it
sets the limit to the stones tested.
The stones with greater refractive
index than the contact liquid cannot
be tested and hence give negative
reading.
- Make sure there is adequate amount
of light. White light can be used
while testing, but monochromatic
yellow light is widely used. White
light can give good results for
single refractive stones, but for
doubly refractive gems sodium light
source is the best option. White
light, when used with doubly
refractive gemstones causes overlap
of the refraction readings thereby
getting wrong results. Also, the use
of sodium light source clearly
differentiates the boundary between
dark and light areas and hence helps
take the readings easily.
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