Vocabulary
• Concentric Fractures• Radial Fractures• Refractive Index• Becke Line• Density
Forensic Analysis of Glass
Why is Glass Analysis important?
• How was it broken?• Link a suspect to the crime• Fingerprints• Blood
Glass Fractures
Concentric
Radial
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Glass Fractures
• Concentric:– A crack in the glass that forms a rough circle
around the point of impact.• Radial:– A crack in the glass that extends outward like the
spoke of a wheel from the point at which the glass was struck.
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Direction of
Penetration
A projectile hole is inevitably wider at the
exit side
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Direction of
Penetration
Concentric
Radial
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Stress MarksThe perpendicular edge of stress marks (shaped
like arches) always face the surface on which the crack originated.
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Successive PenetrationsA fracture always terminates at an existing line of
fracture…which bullet hole was made first?? bsapp.com
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INTERACTIVE
• Draw two bullet holes in a glass window…label: Concentric fractures, Radial Fractures, and which hole was shot first.
Linking Glass to a Suspect…Glass Comparison
• If pieces of class are found on a suspect a CSI can compare that glass to glass found at the crime scene.
• It can either link a suspect to the scene of a crime or eliminate them as a suspect.
Glass Comparison
Physical Examination Density
Refractive Index
Physical Examination
• With the human eye, a hand lens, or a microscope:–Color–Edge thickness–Presence of labels or imprints–Reconstructing (like a puzzle)
Thickness
• Thickness of glass is related to its application• Light bulbs will be thinner than window glass• Window glass will be thinner than glass from a
front door
Density• Glass from the same object (same car window,
same home window, etc) will have the same density—regardless of the size of the pieces---
• Methods of finding Density of Glass:– Mass/Volume with a graduated cylinder– Floatation Method (uses chemicals that are unsafe in a
high school classroom)– Modified Floatation Method (uses density comparison
in water, rubbing alcohol)
Density
• If the densities of the glass on the suspect are different from the glass at the crime scene…then the suspect is eliminated
• If the densities of the glass on the suspect match the crime scene glass than further tests can be done to ensure the glass is a match.
Refractive Index• Any object that transmits light has a
refractive index or a measure of how much the object slows the speed of light.• Refractive Index is the ration of the velocity
of light in a vacuum to light in the particular medium.• Example: Refractive Index of water is 1.33---
light travels 1.33 times faster in a vacuum than it does through water.• When light passes through one medium to
another with a different refractive index, the bending of light occurs (pencil example)
Refractive Index• Every liquid has its own refractive index.• When glass is inserted into a liquid if its refractive
index is the same it will seem to ‘disappear’• FBI has a database of 2000 different types of glass
and their refractive index.• The BECKE LINE is the halo that appears around a
particle in a liquid with a different refractive index.
• Comparison of glass—does it have the same refractive index?
Refraction of Glass Video Clip
• http://www.youtube.com/watch?v=KyWgnFm3ebc
• http://www.youtube.com/watch?v=qH1S83Bkttw
INTERACTIVE
• Make a chart or diagram to summarize the 3 ways to compare and analyze glass fragments.
Assessment
• Glass Analysis Lab