Download Shattered: Forensic Glass Analysis

Shattered: Forensic Glass
Analysis
Glass Evidence
By the end of this chapter you will be able to:
 explain how glass is formed
 list some of the characteristics of glass
 provide examples of different types of
glass
 calculate the density of glass
 use the refractive index to identify
different types of glass
 describe
howSouth-Western
glass /fractures
All Rights Reserved
Cengage Learning © 2009
 analyze glass fracture patterns
2
History of Glass




The earliest human-made glass objects (beads) date
back to about 2500 B.C. Egypt.
Specialized glass production was an art, a science,
and a state secret in the republic of Venice over a
span of hundreds of years.
By the fourteenth century, the knowledge of glass
production spread throughout Europe.
The industrial revolution brought the mass production
of many kinds of glass.
Forensic Science:
Fundamentals &
Investigations, Chapter 14
3
What is Glass?




No specific melting point
Softens over a temperature range.
Uniform amorphous solid (irregular
atomic structure – middle picture).
Because of this, glass breaks in a
variety of fracture patterns.
What’s in Glass?

Formers (form glassy, non-crystalline
structure):
SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5

Fluxes (lowers melting point):
Na2O, K2O, LiO, Al2O3, B2O3, Cs2O

Stabilizers (Chem./Corrosion Resistance):
CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO
Glass Properties



Combinations of formers, fluxes, and
stabilizers creates unique glass types with
different properties:
Density
Refractive Index (RI)
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.
Forensic Science:
Fundamentals &
Investigations, Chapter 14
7
Density Practice (Left Page)




Find the density for the following pieces of
glass:
1. Mass: 4g, Volume Displaced: 2mL
2. Mass: 15g, Volume Displaced: 3mL
3. Mass: 1g, Volume Displaced: 2mL
Refractive Index
 When a beam of light moves from one medium into
another, its speed changes. That change causes the
beam to change direction, bend.
 Measure of how much the speed of light changes
(n>1).
(Air’s RI: n=1)
9
Refractive Index


The direction of the light forms two angles with the
normal .
If the light passes into a denser medium (the gray area),
its direction will bend toward the normal.
10
Populations of Glass RIs
 Because of all the different materials in glass,
there will be multiple RI’s  a major one will
stick out, like above.
Snell’s Law of Refraction






Can figure out the RI of
materials based on the angle
the light hits and bends.
n1sin(θ1) = n2sin(θ2)
n1 = RI of incident light
θ1= angle of incident light
n2 = RI of refracted light
θ2= angle of refracted light
Snell’s Law Example 1





Light traveling in air (n1=1) hits a
piece of glass at an angle of 30
degrees. The light refracts in the
glass at an angle of 15 degrees.
What is the index of refraction
(RI) of the glass?
n1=1, θ1= 30, θ2= 15, n2=?
1sin(30)=n2sin(15)
0.5=0.2588n2
n2 = 1.93
Snell’s Law Example 2






Light traveling in air (n1=1) hits a
piece of glass at an angle of 45
degrees. If the glass has a
refractive index of 1.5, what angle
should the light bend at?
n1=1, θ1= 45, θ2= ?, n2=1.5
1sin(45)=1.5sin(θ2)
0.7071=1.5sin(θ2)
.4714= sin(θ2)
θ2= 27 degrees
Snell’s Law Practice




1. Light traveling in air (n1=1) hits a piece of glass at an
angle 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?
2. Light traveling in air (n1=1) hits a piece of glass at an
angle 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?
3. Light traveling in air (n1=1) hits a piece of glass at an
angle of 25 degrees. If the glass has a refractive index of
1.75, what angle should the light bend at?
4. Light traveling in air (n1=1) hits a piece of glass at an angle
of 55 degrees. If the glass has a refractive index of 1.3, what
angle should the light bend at?