PHYSICAL PROPERTIES GLASS. Physical vs. Chemical Properties The forensic scientist must constantly determine properties that uniquely describe a piece.
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PHYSICAL PROPERTIESGLASS
Physical vs. Chemical Properties
• The forensic scientist must constantly determine properties that uniquely describe a piece of evidence.
• Physical properties such as weight, volume, color, boiling point, and melting point describe a substance without reference to any other substance.
• A Chemical property describes the behavior of a substance when it reacts or combines with another substance.
Important Physical Properties
• Density is defined as the mass per unit volume (D = M/V)
• Density is an intensive property of matter, meaning it remains the same regardless of sample size.
• It is considered a characteristic property of a substance and can be used as an aid in identification.
Determination of Density
• Remember your density lab from chemistry?
• A straight mathematical model of calculating density will seldom work with evidence brought into the laboratory.
• Broken fragments of glass and even plastic are irregularly shaped and their volumes are not accurately measurable.
Flotation Method
• The flotation method is a precise and rapid method for comparing glass densities.
• A glass particle is immersed in a liquid of known density.
• The density of the liquid is adjusted by addition of small amounts of an appropriate liquid until the glass chip remains suspended in liquid.
Refraction
• Light waves travel at a constant speed until they encounter a new medium such as glass or water.
• The new medium will slow down the light causing the wave to bend or refract.
• The amount of this refraction is dependent on the ratio between the speed of light in the two mediums.
Seen Phenomena
Refractive Index
• Refractive index is a distinguishable physical property of glass.
• Refractive index is a value always greater than 1.
• For example the index of refraction for water is 1.3; meaning that light travels 1.3 faster in air than water.
• It is calculated by taking the ratio between the speed of light in air to the speed of light in the medium.
Index of Refraction
Immersion Method
• One method for determining the refractive index is immersing it in a liquid with a similar index.
• With the immersion method the analyst is looking for the Becke Line which is a bright halo surrounding the piece of glass.
• The index of the liquid is adjusted with temperature until this line disappears.
Becke Lines around Glass Crystals
Glass Structure
What is Glass?
• Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides.
• Amorphous solids have their atoms arranged randomly, unlike crystals.
• The added metal oxides provide various properties to glass and also make them unique.
Common Glass Types
• Tempered glass is stronger than normal glass due to rapid heating and cooling.
• Tempered glass is used in car windows.
• Laminated glass found in car windshields has a layer of plastic between two pieces of ordinary window glass.
• Most homes now are required to use tempered glass for safety.
Broken Tempered Glass
Analyzing Cracks
• The penetration of window glass by a projectile, whether it is a bullet or a stone, produces cracks which radiate outward (radial fractures) and encircle the hole (concentric fractures).
• By analyzing the radial and concentric fracture patterns in glass, the forensic scientist can determine the direction of impact.
Radial and Concentric Fractures
Analyzing Cracks
• A high-velocity projectile such as a bullet often leaves a hole that is wider at the exit side, and hence its examination is important in determining the direction of impact.
• The sequence of impacts when there have been successive penetrations of glass is frequently possible to determine because a fracture always terminates at an existing line of fracture.
Which bullet hole was first?
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