Shattered: Forensic Glass Shattered: Forensic Glass AnalysisAnalysis

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Glass Evidence Glass Evidence you will be able to:you will be able to:

explain how glass is formed explain how glass is formed list some of the characteristics of glass list some of the characteristics of glass provide examples of different types of provide examples of different types of

glass glass calculate the density of glass calculate the density of glass use the refractive index to identify use the refractive index to identify

different types of glass different types of glass describe how glass fractures describe how glass fractures analyze glass fracture patterns analyze glass fracture patterns All Rights Reserved South-Western / Cengage Learning ©

2009

Forensic Science: Fundamentals & Investigations, Chapter 14 3

History of Glass History of Glass The earliest human-made glass objects (beads) The earliest human-made glass objects (beads)

date back to about 2500 B.C. Egypt. date back to about 2500 B.C. Egypt. Specialized glass production was an art, a Specialized glass production was an art, a

science, and a state secret in the republic of science, and a state secret in the republic of Venice over a span of hundreds of years. Venice over a span of hundreds of years.

By the fourteenth century, the knowledge of By the fourteenth century, the knowledge of glass production spread throughout Europe. glass production spread throughout Europe.

The industrial revolution brought the mass The industrial revolution brought the mass production of many kinds of glass. production of many kinds of glass.

What is Glass?What is Glass? No specific melting pointNo specific melting point Softens over a temperature Softens over a temperature

range.range. Uniform amorphous solid Uniform amorphous solid

(irregular atomic structure – (irregular atomic structure – middle picture).middle picture).

Because of this, glass breaks in a Because of this, glass breaks in a variety of fracture patterns. variety of fracture patterns.

What’s in Glass? What’s in Glass?

FormersFormers (form glassy, non-crystalline (form glassy, non-crystalline structure):structure):SiOSiO22, B, B22OO33, P, P22OO55, GeO, GeO22, V, V22OO55, As, As22OO33, Sb, Sb22OO55

FluxesFluxes (lowers melting point): (lowers melting point):NaNa22O, KO, K22O, LiO, AlO, LiO, Al22OO33, B, B22OO33, Cs, Cs22OO

StabilizersStabilizers (Chem./Corrosion (Chem./Corrosion Resistance):Resistance):CaO, MgO, AlCaO, MgO, Al22OO33, PbO, SrO, BaO, ZnO, ZrO, PbO, SrO, BaO, ZnO, ZrO

Glass PropertiesGlass Properties

Combinations of formers, fluxes, and Combinations of formers, fluxes, and stabilizers creates unique glass types stabilizers creates unique glass types with different properties:with different properties:

DensityDensity Refractive Index (RI)Refractive Index (RI)

Forensic Science: Fundamentals & Investigations, Chapter 14 7

Density Density

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

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

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

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

Density Practice (Left Page)Density Practice (Left Page)

Find the density for the following Find the density for the following pieces of glass:pieces of glass:

1. Mass: 4g, Volume Displaced: 2mL1. Mass: 4g, Volume Displaced: 2mL 2. Mass: 15g, Volume Displaced: 3mL2. Mass: 15g, Volume Displaced: 3mL 3. Mass: 1g, Volume Displaced: 2mL3. Mass: 1g, Volume Displaced: 2mL

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Refractive Index Refractive Index

When a beam of light moves from one medium into When a beam of light moves from one medium into another, its speed changes. That change causes the beam another, its speed changes. That change causes the beam to change direction, bend. to change direction, bend.

Measure of how much the speed of light changes (n>1).Measure of how much the speed of light changes (n>1).(Air’s RI: n=1)(Air’s RI: n=1)

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Refractive Index Refractive Index

The direction of the light forms two angles with the The direction of the light forms two angles with the normal . normal .

If the light passes into a denser medium (the gray If the light passes into a denser medium (the gray area), its direction will bend toward the normal. area), its direction will bend toward the normal.

Populations of Glass RIs

Because of all the different materials in Because of all the different materials in glass, there will be multiple RI’s glass, there will be multiple RI’s a major a major one will stick out, like above.one will stick out, like above.

Snell’s Law of RefractionSnell’s Law of Refraction

Can figure out the RI of Can figure out the RI of materials based on the angle materials based on the angle the light hits and bends.the light hits and bends.

nn11sin(θsin(θ11) = n) = n22sin(θsin(θ22)) nn1 1 = RI of incident light= RI of incident light θθ11= angle of incident light = angle of incident light nn2 2 = RI of refracted light= RI of refracted light θθ22= angle of refracted light = angle of refracted light

Snell’s Law Example 1Snell’s Law Example 1 Light traveling in air (nLight traveling in air (n11=1) hits a =1) hits a

piece of glass at an angle of 30 piece of glass at an angle of 30 degrees. The light refracts in the degrees. The light refracts in the glass at an angle of 15 degrees. glass at an angle of 15 degrees. What is the index of refraction What is the index of refraction (RI) of the glass?(RI) of the glass?

nn11=1, θ=1, θ11= 30, θ= 30, θ22= 15, n= 15, n22=?=? 1sin(30)=n1sin(30)=n22sin(15)sin(15) 0.5=0.2588n0.5=0.2588n22

nn22 = 1.93 = 1.93

Snell’s Law Example 2Snell’s Law Example 2 Light traveling in air (nLight traveling in air (n11=1) hits a =1) hits a

piece of glass at an angle of 45 piece of glass at an angle of 45 degrees. If the glass has a degrees. If the glass has a refractive index of 1.5, what angle refractive index of 1.5, what angle should the light bend at?should the light bend at?

nn11=1, θ=1, θ11= 45, θ= 45, θ22= ?, n= ?, n22=1.5=1.5 1sin(45)=1.5sin(θ1sin(45)=1.5sin(θ22)) 0.7071=1.5sin(θ0.7071=1.5sin(θ22)) .4714= sin(θ.4714= sin(θ22)) θθ22= 27 degrees= 27 degrees

Snell’s Law PracticeSnell’s Law Practice 1. Light traveling in air (n1. Light traveling in air (n11=1) hits a piece of glass at an angle =1) hits a piece of glass at an angle

of 37 degrees. The light refracts in the glass at an angle of of 37 degrees. The light refracts in the glass at an angle of 30 degrees. What is the index of refraction (RI) of the glass?30 degrees. What is the index of refraction (RI) of the glass?

2. Light traveling in air (n2. Light traveling in air (n11=1) hits a piece of glass at an angle =1) hits a piece of glass at an angle of 60 degrees. The light refracts in the glass at an angle of of 60 degrees. The light refracts in the glass at an angle of 15 degrees. What is the index of refraction (RI) of the glass?15 degrees. What is the index of refraction (RI) of the glass?

3. Light traveling in air (n3. Light traveling in air (n11=1) hits a piece of glass at an angle =1) hits a piece of glass at an angle of 25 degrees. If the glass has a refractive index of 1.75, of 25 degrees. If the glass has a refractive index of 1.75, what angle should the light bend at?what angle should the light bend at?

4. 4. Light traveling in air (nLight traveling in air (n11=1) hits a piece of glass at an angle of =1) hits a piece of glass at an angle of 55 degrees. If the glass has a refractive index of 1.3, what angle 55 degrees. If the glass has a refractive index of 1.3, what angle should the light bend at?should the light bend at?