Chapter 9: Glass Examination Flashcards
The Todd Christensen was solved through what type of analysis?
Glass and plastic analysis
How did they solve the Todd Christensen case?
- They received a tip from a citizen who knew the culprit and who thought he was the suspect based on the car he drove
- They looked into John (suspect) who denied that it was him in the hit and run
- They gained access to his garage and found a broken piece of orange plastic on his work bench
- This plastic casing of a turn signal matched up to the pieces found at the scene of the crime
What comparative analysis did they do to solve the Todd Christensen case?
- Paint chips and plastic signal chips from scene of crime compared to the ones found in his garage on the workbench
What is a physical property?
Physical properties describe a substance without reference to any other substance.
What are examples of physical properties?
- Weight
- Volume
- Colour
- Boiling point
- Melting point
What is a chemical property?
A chemical property describes the behavior of a substance when it reacts or combines with another substance.
What is matter?
Matter is anything that has a mass and occupies space.
What is an element?
An element is the simplest substance known and provides the building block from which all matter is composed.
What is a compound? (in chemistry)
When elements combine
Give an example of a compound
- NaCl
- NaOH
- KOH
- H2O
- CO2
etc
What is an atom?
- An atom is the basic particle of an element
What is a molecule?
- molecule is the smallest unit of a compound.
- a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction.
What are the 3 states of matter?
- Solid
- Liquid
- Gas
Define a solid.
Definite shape and volume
Define a liquid.
Specific volume, takes the shape of its container
Define a gas.
Neither a definite shape nor volume
What system of measurement do scientists throughout the world use?
Metric system
What are the basic units of measurement for length, mass, and volume in the metric system?
- Meter
- Gram
- Liter
What are common prefixes used in the metric system?
- Deci
- Centi
- Milli
- Micro
- Nano
- Kilo
- Mega.
What is temperature? (Definition)
Temperature is a measure of heat intensity, or the hotness or coldness of a substance.
What is the most commonly used scale for temperature?
Celcius
What is the freezing and boiling point of water?
Freezing - 0°C
Boiling - 100°C
What is weight? (Definition)
Weight is the force with which gravity attracts a body.
What is mass? (Definition)
Mass refers to the amount of matter an object contains independent of gravity.
What is density?
- Density is defined as the mass per unit volume
- Density is an intensive property of matter, meaning it remains the same regardless of sample size.
What is the formula for density?
D = M/V
Density is considered a characteristic property of a substance and can be used as an aid in…
Identification
What are the 2 models to describe the behaviour of light?
- Light is described as a continuous wave
- Light is depicted as a stream of discrete energy particles
What is the electromagnetic spectrum?
The electromagnetic spectrum is the entire range of radiation energy from the most energetic cosmic rays to the least energetic radio waves.
What is a wavelength?
Wavelength, the distance between two successive crests (or one trough to the next trough).
What is frequency?
Frequency, the number of crests (or troughs) passing any one given point per unit of time.
What is refraction?
The bending of light waves because of a change in velocity
What is refractive index?
Refractive index is the ratio of the velocity of light in a vacuum to that in the medium under examination.
At 25°C, what is the refractive index of water?
1.333
What do crystalline solids do?
These solids refract a beam of light in two different light-ray components. This results in double refraction
What is glass composed of?
Silicon oxides (sand) mixed with various metal oxides.
What are the two things added to the sand mix and why?
Sodium carbonate (soda) is normally added to the sand to lower its melting point. Calcium carbonate (lime) is also added to the sand mix to prevent the glass from dissolving in water.
What type of glass in found in car windshields?
Laminated glass
What is tempered glass made up of?
- Ultra fine series of invisible stress lines develop
- It shatters along these many tiny stress lines into small, safer pieces, uniform in size, edges smooth
What do they use tempered glass for?
Used for car side and rear windows
What’s the inside layer made of in laminated glass?
Vinyl
To compare glass fragments, a forensic scientist evaluates what two important physical properties?
- Density
- Refractive index
What is the flotation method used for?
This is a rather precise and rapid method for comparing glass densities
What is the process of the flotation method?
- In the flotation method, a glass particle is immersed in a liquid.
- The density of the liquid is carefully adjusted by the addition of small amounts of an appropriate liquid until the glass chip remains suspended in the liquid medium.
- At this point, the glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float.
What is the glass immersion method used for?
To determine a glass fragment’s refractive index.
What’s the process of the glass immersion method?
- It involves immersing a glass particle in a liquid whose refractive index is varied until it is equal to that of the glass particle.
- At this point, known as the match point, the Becke line disappears and minimum contrast between liquid and particle is observed.
What is the Becke Line?
The Becke line is a bright halo near the border of a particle that is immersed in a liquid of a different refractive index.
What does the Becke Line indicate when inside of the glass vs. outside?
- The Becke line is a “halo” that can be seen on the inside of the glass (left) indicating that the glass has a higher refractive index than the liquid
- The Becke line (right) is outside of the glass, indicating the opposite.
What are radical fractures in regards to glass cracks?
Produces cracks that radiate outward (radial fractures)
What are concentric fractures in regards to glass cracks?
Cracks that encircle the hole
How can you determine the direction of impact of glass cracks?
By analyzing the radial and concentric fracture patterns in glass, the forensic scientist can determine the direction of impact
What is the 3R Rule?
Radial cracks form a Right angle on the Reverse side of the force.
What type of packaging should glass fragments be packaged in?
Solid containers to avoid further breakage
If the suspect’s shoes and/or clothing are to be examined for the presence of glass fragments, how should they be packaged?
They should be individually wrapped in paper and transmitted to the laboratory.
How can you tell the entry and exit hole in glass from a bullet?
The entry side of a bullet going through glass is smaller than the exit
How can you tell the order of different bullet holes in glass?
You can tell the order of different bullet holes by the radial fracture
