4-1 prentice hall ©2008 pearson education, inc. upper saddle river, nj 07458 forensic science an...

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PRENTICE HALL ©2008 Pearson Education, Inc. Upper Saddle River, NJ 07458

FORENSIC SCIENCE An IntroductionBy Richard Saferstein

THE ANALYSIS OF GLASS

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Glass Fragments• Glass is a hard, brittle, amorphous

substance that is composed of silicon oxides mixed with various metal oxides.

• Amorphous solids have their atoms arranged randomly, unlike crystals.

• Tempered glass is stronger than normal glass due to rapid heating and cooling.

• Laminated glass found in car windshields has a layer of plastic between two pieces of ordinary window glass.

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Glass Fragments• For the forensic scientist, the problem of

glass comparison is one that depends on the need to find and measure those properties that will associate one glass fragment with another while minimizing or eliminating other sources.

• To compare glass fragments, a forensic scientist evaluates two important physical properties: density and refractive index.

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Important Physical Properties• Density is defined as the mass per unit

volume. (D = M/V)– Density is an intensive property of matter,

meaning it remains the same regardless of sample size.

– It is considered a characteristic property of a substance and can be used as an aid in identification.

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Forensic Science: Fundamentals & Investigations, Chapter 14 5

Density

The formula for calculating density is: D = m / V

The mass (m) of a fragment of glass can be found using a balance beam device.

Place the fragment of glass into a beaker filled with water and measure the volume (V) of overflow.

Divide the mass (in grams) by the volume (in milliliters) to find the density (D) of the glass fragment in g/ml

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Density PracticeFind the density for the following pieces

of glass:1. Mass: 4g, Volume Displaced: 2mL2. Mass: 15g, Volume Displaced: 3mL3. Mass: 1g, Volume Displaced: 2mL

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Important Physical Properties• Light waves travel in air at a constant

speed until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend.

• The bending of light waves because of a change in velocity is called refraction.

• Refractive index is the ratio of the speed of light in a vacuum to that in the medium under examination.

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Important Physical Properties• For example, at 25oC the refractive index

of water is 1.333.

• This means that light travels 1.333 times faster in a vacuum than it does in water.

• Like density, refractive index is an intensive property and will serve to characterize a substance.

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Figure 4–10 Light is refracted when it travels obliquely from one medium to another.

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Figure 4-4 The electromagnetic spectrum.

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Figure 4–3 Representation of the dispersion of light by a glass prism.

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Analyzing Cracks• The penetration of window glass by a

projectile, whether it is a bullet or a stone, produces cracks which radiate outward (radial fractures) and encircle the hole (concentric fractures).

• By analyzing the radial and concentric fracture patterns in glass, the forensic scientist can determine the direction of impact.

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Fractures

Conce

ntri

c

Rad

ial

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FORENSIC SCIENCE An IntroductionBy Richard Saferstein bsapp.com

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FORENSIC SCIENCE An IntroductionBy Richard Saferstein bsapp.com

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Direction of

Penetration

A projectile hole is always wider at the

exit side

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Direction of

Penetration

ConcentricConcentric

RadialRadial

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Stress MarksThe perpendicular edge of

stress marks always face the surface on which the crack

originated

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Analyzing Cracks• The direction of impact can also be

accomplished by applying the 3R Rule: Radial cracks form a Right angle on the Reverse side of the force.

• It is frequently possible to determine the sequence of impacts when there have been successive penetrations of glass because a fracture always terminates at an existing line of fracture.

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A B

Which hole was made first?

4-1 prentice hall ©2008 pearson education, inc. upper saddle river, nj 07458 forensic science an...

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