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 bsapp.com

Glass Fractures Concentric: Radial: 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 bsapp.com

Direction of Penetration Radial Concentric bsapp.com

Stress Marks The perpendicular edge of stress marks (shaped like arches) always face the surface on which the crack originated. bsapp.com

Successive Penetrations A 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 Insert a link for You Tube Video?

INTERACTIVE Make a chart or diagram to summarize the 3 ways to compare and analyze glass fragments.

Assessment Glass Analysis Lab Foldable of some kind??