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