Forensic Glass Analysis:
Refractive Index of Glass Fragments
Goal
The final step in your analysis is to determine the refractive index of the glass fragment
evidence.
Method
When light travels from one medium to another of different density, it does not travel in a
straight line, but bends at the boundary between the two mediums. This bending of light is called
“refraction“. Light refracts because the density difference between the two mediums changes the
velocity of the light. Light bends and slows down when it travels from a low density to a high
density material and speeds up when it travels from a high density material to a low density
material.
Light rays in liquids can be used to determine refraction of glass types. Each glass type
has a signature refraction index. When a sample of glass is placed in a liquid the sample may or
may not be visible. Becke Lines measurements can be used as a relative measure of a sample’s
visibility. If the outside perimeter of the sample is clearly visible in the liquid, then it has a
different refractive index than the liquid and bends the light (see Figure 6a). If the sample
disappears in the liquid, then it has a refractive index similar to the liquid and does not bend the
light.
In this lab different types of glass and refraction oils can be compared under the
microscope by observing the different glass in different refraction oils. Thus, forensic scientists
can further identify the glass, connecting the glass to a suspect. You will analyze four sample
fragments of glass and a fifth sample fragment collected from a suspect.
Prelab/Questions: (Write the answers on your own notebook paper.)
1. What is refractive index? Draw a picture illustrating what happens to light when it passes
through 2 different mediums. Make sure you label all relevant parts.
2. What is the purpose of this lab (ie. What forensic science concepts are you being asked to
explore and learn?
3. Briefly describe what you are being asked to do in this lab.
4. Create a data table.
Procedure:
1. Clean microscope slides with soapy water and rinse well with distilled water and dry
completely with Kimwipe or lens paper. Label appropriately with the liquid and the glass
type or index.
2. Sodium chloride crystals can be compared in the above refractive liquids with a clear
Becke line. Sodium chloride is regular in shape while the glass fragments look like
boulders (i.e. irregular shapes) under the microscope.
3. Using clean forceps, place a very small amount (as little as possible) of glass fragment
onto the labeled slide.
4. Add the refractive liquid of choice and swirl gently.
5. Draw a picture of each type of glass fragment in each type of refractive liquid.
6. Compare as many combinations of glass types and refractive liquids as possible, looking
for the Becke line. The larger the differences between the refractive indices of the glass
and the liquid, the darker and clearer the Becke line.
7. Determine the refractive index range (closest to which liquids i.e. lightest Becke line) of
the glass sample.
8. By comparing different glass samples, a match of known glass to the glass found at a
crime scene can be determined.
Refractive liquid
water
Refractive Index
1.330
Isopropyl alcohol
1.377
olive oil
1.4667
castor oil
1.4820
Type of Glass
Pyrex
Refractive Index
1.45
Headlight glass
Bottle Glass
1.47
1.51
Post lab Questions: (Write the answers on your own notebook paper.)
1. How would you use refractive liquids to identify different types of glass?
2. What is the Becke line?
3. In which medium and with which type of glass was it easiest to see the Becke line and
why?
4. Explain why different types of glass have different refractive indices.