Shattered: Forensic Glass Analysis

2 Glass Evidence 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 glass provide examples of different types of glass calculate the density of glass calculate the density of glass use the refractive index to identify different types of glass use the refractive index to identify 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 143 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.

What is Glass? No specific melting point No specific melting point Softens over a temperature range. Softens over a temperature range. Uniform amorphous solid (irregular atomic structure – middle picture). Uniform amorphous solid (irregular atomic structure – middle picture). Because of this, glass breaks in a variety of fracture patterns. Because of this, glass breaks in a variety of fracture patterns.

What’s in Glass? Formers (form glassy, non-crystalline structure): Formers (form glassy, non-crystalline structure): SiO 2, B 2 O 3, P 2 O 5, GeO 2, V 2 O 5, As 2 O 3, Sb 2 O 5 Fluxes (lowers melting point): Fluxes (lowers melting point): Na 2 O, K 2 O, LiO, Al 2 O 3, B 2 O 3, Cs 2 O Stabilizers (Chem./Corrosion Resistance): Stabilizers (Chem./Corrosion Resistance): CaO, MgO, Al 2 O 3, PbO, SrO, BaO, ZnO, ZrO

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

Forensic Science: Fundamentals & Investigations, Chapter 147 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. 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. 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. Divide the mass (in grams) by the volume (in milliliters) to find the density (D) of the glass fragment.

Density Practice (Left Page) Find the density for the following pieces of glass: Find the density for the following pieces of glass: 1. Mass: 4g, Volume Displaced: 2mL 1. Mass: 4g, Volume Displaced: 2mL 2. Mass: 15g, Volume Displaced: 3mL 2. Mass: 15g, Volume Displaced: 3mL 3. Mass: 1g, Volume Displaced: 2mL 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. 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) Measure of how much the speed of light changes (n>1). (Air’s RI: n=1)

10 Refractive Index The direction of the light forms two angles with the normal. 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. 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. 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. Can figure out the RI of materials based on the angle the light hits and bends. n 1 sin(θ 1 ) = n 2 sin(θ 2 ) n 1 sin(θ 1 ) = n 2 sin(θ 2 ) n 1 = RI of incident light n 1 = RI of incident light θ 1 = angle of incident light θ 1 = angle of incident light n 2 = RI of refracted light n 2 = RI of refracted light θ 2 = angle of refracted light θ 2 = angle of refracted light

Snell’s Law Example 1 Light traveling in air (n 1 =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? Light traveling in air (n 1 =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? n 1 =1, θ 1 = 30, θ 2 = 15, n 2 =? n 1 =1, θ 1 = 30, θ 2 = 15, n 2 =? 1sin(30)=n 2 sin(15) 1sin(30)=n 2 sin(15) 0.5=0.2588n 2 0.5=0.2588n 2 n 2 = 1.93 n 2 = 1.93

Snell’s Law Example 2 Light traveling in air (n 1 =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? Light traveling in air (n 1 =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? n 1 =1, θ 1 = 45, θ 2 = ?, n 2 =1.5 n 1 =1, θ 1 = 45, θ 2 = ?, n 2 =1.5 1sin(45)=1.5sin(θ 2 ) 1sin(45)=1.5sin(θ 2 ) =1.5sin(θ 2 ) =1.5sin(θ 2 ).4714= sin(θ 2 ).4714= sin(θ 2 ) θ 2 = 27 degrees θ 2 = 27 degrees

Snell’s Law Practice 1. Light traveling in air (n 1 =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? 1. Light traveling in air (n 1 =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 (n 1 =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? 2. Light traveling in air (n 1 =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 (n 1 =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? 3. Light traveling in air (n 1 =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 (n 1 =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? 4. Light traveling in air (n 1 =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?