Spectroscope @ Jewel Info 4 U
By:Ritika Changrani
Summary: A Spectroscope helps
to identify cut stones, rough
stones, mounted stones and unmounted
stones to see whether they are real
or fakes.
A Spectroscope is a gemological
device used to test if the gemstone
is natural or synthetic.
Spectroscope helps to determine what
parts of white light are being
absorbed by the gemstone.
White light is made up of seven
different colours namely violet,
indigo, blue, green, yellow, orange
and red, which have different
wavelengths and travel with
different speed. When white light is
passed through a gemstone, a part of
it, based on the energy level is
absorbed by the gemstone and the
remaining colours combine to give a
different colour to the stone. When
observed through the spectroscope,
one can see bands or colours
missing. The missing bands are the
colours absorbed by the gemstone.
The colours absorbed let us know the
elements present and the chemical
composition in the gemstone which
thereby helps in gemstone
identification.
Types of Spectroscope
There are two types of spectroscope,
diffraction grating spectroscope and
prism spectroscope.
Diffraction Grating Spectroscope
Diffraction grating spectroscope
implies the principle of
diffraction. Diffraction refers to
the bending of light waves around a
sharp edge or an obstacle by
transmission or by reflection. This
type of spectroscope is made up of
high-dispersion diffraction grating
film, movable slits and a photo
detector which is used to measure
the properties of light within the
specified portion of the spectrum.
Here light is made to enter a narrow
slit. The light is then diffracted
by thin high-dispersion diffraction
grating film. This causes uniform
spectrum image and disperses light
into large visible spectrum. The
advantage of diffraction grating
spectroscope over the prism
spectroscopes is that the spectrum
is evenly distributed and hence is
easier to read. But the spectrum
thus produced is not as bright as
that produced by prism spectroscope.
Diffraction grating spectroscopes do
not have an inbuilt calibration
scale.
Prism Spectroscope
Prism spectroscope works on the
principle of dispersion. Dispersion
is defined as the splitting of white
light into its constituent colours.
Prism spectroscope is made of three
optical grade glass prisms placed in
optical contact with each other.
Most of the prism spectroscopes come
with calibration scale. But
experienced people make observations
without the scale.
In a prism spectroscope, light is
made to enter through a narrow slit
which is dispersed by passing
through a series of prisms. In prism
spectroscopes, the spectrum obtained
is brighter and faint lines are
clearly visible. But the
disadvantage with prism spectroscope
is that the spectrum is not evenly
distributed. Blue parts are more
spread and red parts are more
condensed. It is difficult to
distinguish lines in red part since
they are very close to each other.
Prism spectroscopes come with focus
slide control and light slit control
which allows adjusting for the
amount of light entering the unit.How to use a Spectroscope?-
First of all, before testing for
the absorption spectra of the
gemstones, hold the spectroscope
against some different sources of
illumination. Holding it against
either a fluorescent light bulb or a
computer monitor shows clear
absorption bands.
- Place the gemstone on a black
non-reflective surface may be a
piece of black velvety cloth. If the
surface is non-reflective, the
observations may not be accurate and
hence give false readings.
- Place the source of white light
such that light enters the pavilion
of gemstone at an angle of 45
degrees.
- Place the spectroscope on the
other side at the same angle. The
angle of 45 degree is chosen since
this is the path in which light
travels in the longest path picking
up most colours.
- The other way of positioning can
be by placing the light source and
the gemstone in one line such that
the light illuminates the gemstone
from the back. This helps to view
the gemstone in transmitted light.
- Note the observations through the
spectroscope. Note down the spectrum
seen with red end of the spectrum on
the left and violet end of the
spectrum to the right. The colours
which are absorbed by the gemstone
appear as missing bands. The colours
absorbed mix up to give the gemstone
a different colour.
- Based on the spectrum obtained,
you can analyze the composition of
the gemstone and hence determine the
type of gemstone.
- The spectrum can also be compared
with that of a known gemstone to
identify if the gemstone is a
natural or a synthetic one.
Though spectroscope is one of the
important tools in gemology, it
should be purchased only when it is
extremely necessary. The cost of
spectroscope is quite high.
Furthermore, only experienced
gemmologists can make use of the
spectroscope more efficiently. It is
difficult for an inexperienced
person to study the nature of
gemstones using a spectroscope.
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