1. Shattered: Forensic Glass Analysis

2. 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 how glass fractures
analyze glass fracture patterns
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3. 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

4. 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.
Crystalline solids have a regular atomic structure (illustrated above, top).

5. What’s in Glass? SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5
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

6. Glass Properties
Combinations of formers, fluxes, and stabilizers creates unique glass types with different properties:
Density
Refractive Index (RI)

7. 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

8. 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

9. 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)
Refractive index is a # greater than one because it is a ratio of the speed of light (3e8) divided by the speed in the glass/material (something lower than 3e8).

10. 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.

11. 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.

12. 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

13. 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

14. 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

15. 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?