Forensic Science: Glass Identification
Purpose: To estimate the refractive index of glass fragments and to compare the
refractive indices of several types of glass.
Forensic scientists use both density and refractive index to match glass fragments from
crime scenes and suspects. Density is estimated by suspending the fragment in solutions
of different densities in order to find the solution with density equal to that of the
fragment, so the fragment neither sinks nor rises. The solutions used are smelly and
toxic, so we will omit this activity.
We have a series of oils with different refractive indices, and microscope slides with
wells. When a fragment is placed in an oil with refractive index which exactly matches,
we can't see the fragment. In order to speed the selection of the correct oil, we will use
the Becke line method. The Becke line s a bright line which is seen at the very edge of
the fragment. Usually about 100 X is the best magnification to use. When you move the
focus up and down, you will see the Becke line move. Practice until you can see this.
First choose an oil with RI near the middle of the range. You can tell if the RI of your
fragment is higher or lower than the oil by watching the Becke line. When you increase
the distance between your eye and the object (by moving the stage down), the Becke line
moves toward the region of higher RI. Decide how this corresponds to the focus knob on
the microscope. Note whether the oil you chose first has a higher or lower RI than the
glass. Thus you can choose the next oil to try. While you have that oil, you might try
several other fragments, but be sure not to mix the fragments up.
When you are ready to try another oil, rinse your fragment with methanol and dry with a
tissue and heat lamp. Students may want to trade microscope slides with the diferent oils
The class can decide which glass matches the unknown fragment.
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Report:
Make a short introduction describing the Becke line. The main part of the report should
be a table of glass types and refractive index. State which glass probably corresponds to
the unknown.
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