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 how glass fractures analyze glass fracture patterns All Rights Reserved South-Western / Cengage Learning © 2009

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

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).

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

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

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) 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).

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.

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?