it is about how abbe's refractometer helps in the forensic analysis of the glass.

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By,
Gopika Babu
1st
Year MSc. Forensic
science Lovely
Professional University
Abbe’s
Refractometer

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The Abbe instrument is the most convenient and
widely used refractometer. Its original design was so successful that even
as of today it is over 150 years old, it is still used and copied in new
devices. Over a century after Abbe's work, the usefulness and precision
of refractometers has improved, although their principle of operation has
changed very little. They are also possibly the easiest device to use for
measuring the refractive index of solid samples, such as glass, plastics,
and polymer films. Some modern Abbe refractometers use a digital
display for measurement, eliminating the need for discerning between
small graduations. However, the user still has to adjust the view to get a
final reading.
It is working on the principle of critical angle. Critical angle is
the incident angle at which light - instead of getting to the other side of
phase boundary - gets refracted in such a way that it becomes parallel to
the phase boundary surface. For smaller incident angles rays get through
the boundary, for larger they get reflected back.
Introduction

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Refractive index so called index of refraction,
measure of the bending of a ray of light when passing from one medium
into another. If i is the angle of incidence of a ray in vacuum (angle
between the incoming ray and the perpendicular to the surface of a
medium, called the normal) and r is the angle of refraction (angle
between the ray in the medium andthe normal),the refractive index n is
defined as the ratio of the sine of the angle of incidence to the sine of
the angle of refraction;i.e., n = sin I / sin r. Refractive index is also equal
to the velocity of light c of a given wavelength in empty space divided
by its velocity v in a substance, or n = c/v.
In forensic science, in analysis of glass refractive index of the
glass and liquid is done by Abbe’s Refractometer.
Abbe’s refractometer was the first refractometer to be offered
commercially. It is worked on the principle critical angle. Critical angle is
Refractive Index of Glass
Abbe’s Refractometer

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the incident angle at which light - instead of getting to the other side of
phase boundary - gets refracted in such a way that it becomes parallel to
the phase boundary surface. For smaller incident angles rays get through
the boundary, for larger they get reflected back.
Critical angle can be easily calculated if we know refractive indices of
both media. We start with the Snell's law:
α is the incident angle, β is the refracted angle. When refracted light gets
parallel to the surface, refraction angle is 90°. That means sin(β) is 1 and
critical angle can be found from
 Sample is put between two prisms - measuring and illuminating.
 Light enters sample from the illuminating prism, gets refracted at
critical angle at the bottom surface of measuring prism, and then the
telescope is used to measure position of the border between bright and
light areas.
 Knowing the angle and refractive index of the measuring prism it is not
difficult to calculate refractive index of the sample.
 Surface of the illuminating prism is matted, so that the light enters the
sample at all possible angles, including those almost parallel to the
surface. By measuring the angle between the shadow boundary and the

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normal to the second face of the prism, the index of the sample may be
calculated.
Refraction in the prism - light source far to the left. Refraction in the prism - light source below prism.
Refraction in the prism - light source on the right. Refraction in the prism - combining various light sources.
Refraction in the prism - light exiting upper prism surface. Refraction in the prism - what can be seen.

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Glass possesses extremely great evidential value as it can
be individualized and its origin can be specified.
Comparing glass comprises of analyzing and measuring the
properties of glass fragment with another having the similar composition
and properties in order to yield a positive match. The physical properties
of glass that successfully characterize the glass particles are density and
refractive index. They are class characteristics thus aid in narrowing down
the possibilities; they do provide sufficient data to evaluate the
significance of glass comparison.
Various factors that affect refractive index measurement are
1. Temperature
Temperature influences the refractive index of a medium primarily
because of the accompanying change in density. For many liquids, the
temperature coefficient lies in the range of -4 to -6 x10-4 deg-1. Water is
an important exception, with a coefficient of about -1 x10-4 deg-1.
2. Wavelength of light used.
The refractive index of a transparent medium gradually decreases with
increasing wavelength; this effect is referred to as normal dispersion. In
the vicinity of absorption bands, rapid changes in refractive index occur;
here the dispersion is anomalous.
3. Pressure
The refractive index of a substance increases with pressure because of the
accompanying rise in density. The effect is most pronounced in gases,
where the change in n amounts to about 3x10-4 per atmosphere; the figure
is less by a factor of 10 for liquids, and it is yet smaller for solids.
Glass
Factors affecting Refractive Index

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 It can be used to measure both refractive index of liquids and solids.
 In both cases refractive index of the substance must be lower than the
refractive index of the glass used to made measuring prism.
 Long and difficult thermostating (requires external water bath)
 Operator dependent readings, therefore limited accuracy.
 No measurement protocol (not suitable for GLP/GMP)
 Tedious calibration.
On a dark, cloudy night in February, at 9:30 pm, a 19-
year-old named Craig Elliott Kalani went for a walk with his dog in his
neighborhood but he never returned because a hit and run driver killed
him. Pieces of glass were found on Craig's jacket and around his body at
the crime scene. These pieces of glass helped crime scene investigators
figure out what happened.
Police searched for a vehicle that had the same damage as the hit and run
car. Then they found a car with those damages that belonged to Susan
Nutt. In order to connect Susan's car to the glass fragments found at the
crime scene to the glass on her car, refractive index tests needed to be
done. This job was done by researchers at Oregon State University's
Radiation Center. The scientists there found that the windshield glass
Advantages
Disadvantages

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contained the same 22 chemicals as the ones in Susan's car. They also said
that both samples of glass were a definite match.
The glass evidence helped convict Susan Nutt. She failed to preform
duties of a driver for an injured person. She was sentenced to 5 years
in jail.
From all these we can conclude that analysis of glass plays an important
role in forensic investigation as well as determination of refractive index
helps the analysis more significant and accurate.
Bibliography
 http://www.refractometer.pl/Abbe-refractomete
 https://vlab.amrita.edu/?sub=1&brch=195&sim=545&cnt=1
 https://www.britannica.com/science/refractive-index