Plastics, Density and Refractive Index Determinations
The characterization of physical evidence, such as glass and plastic fragments, can often be
critical to a successful forensic investigation.
When glass or plastic is broken, such as in an
automobile accident or burglary, fragments are frequently carried away from the scene of the crime
on the clothing, shoes or hair of a suspect.
Both glass and plastics are so ubiquitous in our society
that many crimes involve either their utilization as a weapon or as evidence from the crime scene.
These samples may provide either class information, such as the type of glass or plastic involved, or
individual evidence, such as that obtained by uniquely reconstructing a shattered item.
pieces of evidence may lead to some rather important discoveries, for example;
In 1988, a number of jars of Gerber baby food were discovered to contain
While it was possible that the contamination arose from a
single event, such as a light bulb bursting over the production line, careful
analysis showed these glass fragments arose from a number of quite different
sources (headlight, plate glass, light bulb, etc.).
The conclusion by the FBI was
that several consumers independently placed the fragments into the food jars
themselves in the hope of forcing the company to compensate them for the
potentially dangerous contamination.
It is often important to determine, if possible, whether a particular sample of glass or plastic
found on a suspect matches, at least in terms of chemical composition, a similar material found at
the scene of a crime.
While most glasses look very similar by visual inspection, they may be quite
different in terms of their physical and chemical properties.
Similarly, while plastics may be
colored materials, two colored samples may also be hard to distinguish visually, especially in small
A variety of methods have been employed in determining the specific
physical and chemical properties of glass and plastic samples including, density, refractive index,
ultraviolet-visible and infrared spectroscopy, inductively couple mass spectrometry (IC-MS), and
Especially important among these are density and refractive index
measurements for the substance.
In this lab we will explore some of the methods that may be employed in determining the
density and refractive index of a variety of transparent materials.
The objective of this laboratory is to determine the densities and refractive indices of a
variety of glass and plastic samples using several standard methods.
Many different types of glasses and plastics are manufactured and used for a variety of
Each type of glass, depending upon its composition, has a characteristic density and
Accurately determining these two properties can provide important data about the
similarity or dissimilarity of two samples.