Chapter 4

List and explain forensic methods for comparing glass fragments

Composition of Glass: Theres tempered glass, glass to which strength is added by introducing stress through rapid heating and cooling of the glass surfaces Laminated glass- Two sheets of ordinary glass bonded together with a plastic film. Comparing Glass Fragments Measuring and Comparing Density Determining and Comparing Refractive Index: The Becke Line- A bright halo observed near the border of a particle immersed in a liquid of a different refractive index. Classification of Glass Samples

Understand the differences between the wave and particle theories of light

The wave concept depicts light as having an up-and-down motion of a continuous wave. The wave can be characterised by two distinct properties: wavelength and frequency. Wavelength the distance between crests of adjacent waves. Frequency is the number of waves that pass a given point per unit of time.

Define and distinguish elements and compounds

An element is a fundamental particle of matter that cannot be broken down into simpler substances by chemical means. Periodic table is a chart of all the known elements arranged in a systematic fashion. Atom- is the smallest unit of an element that can exist and still retain its identity as that element. Compound- a pure substance composed of two or more elements An atom is the basic particle of an element and a molecule is the smallest unit of a compound.

Understand and explain the dispersion of light through a prism

Dispersion- The separation of light into its component wavelengths. Once light passes through a prism, each colour component of light is slowed to a speed slightly different from those of the others, causing each component to bend at a different angle as it emerges from the prism. Refraction- The bending of a light wave caused by a change its velocity.

Methods for Testing Glass Fragments

Flotation Method- 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. The immersion method is used to determine a glass fragment's refractive index. It involves immersing a glass particle in a liquid medium 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. The Becke line is a bright halo near the boarder of a particle that is immersed in a liquid of a different refractive index.

Collection of Glass

If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all the glass found. When an individual fit is thought improbable, the evidence collector must submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene. The glass fragments should 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, they should be individually wrapped in paper and transmitted to the laboratory.

Contrast the differences between a solid, liquid and gas

Matter- anything that has mass and occupies space Physical state- the physical form taken by matter: solid, liquid or gas Solid- A state of matter in which the molecules are held closely together by strong attractive forces. It has a definite shape and volume Liquid- A state of matter in which the attractive forces are strong enough to allow molecules to be in contact with one another but too weak to hold them rigidly in place. It has a specific volume but takes the shape of its container. Gas- A state of matter in which the attractive forces between molecules are weak enough to permit them to move with complete freedom. It has no definite shape nor volume. Sublimation- A change of state from a solid directly into a gas. Phases- A uniform substance separated from other substances by definite visible boundaries.

Define and understand the properties of density and refractive index

Physical Properties of Matter: Temperature- Fahrenheit scale is a temperature scale that defines the melting point of ice as 32 degrees and the boiling point of water as 212 degrees, with 180 equal divisions or degrees between. Celsius Scale is the temperature scale that defines the melting point of ice as 0 degrees and the boiling point of water as 100 degrees, with 100 equal divisions or degrees between. Weight and Mass: The force with which gravity attracts a body. Mass: A constant property that refers to the amount of matter an object contains Density: A physical property of matter that is equivalent to the mass per unit volume of a substance. Intensive property- A property that is not dependent on the size of an object. Refractive index- The ratio of the speed of light in a vacuum to its speed in a given medium. Crystalline solid- A solid in which the constituent atoms have a regular arrangement. Amorphous solid- A solid in which the constituent atoms or molecules are arranged in random or disordered positions Birefringence- A difference in the two indices of refraction exhibited by most crystalline materials.

Define and distinguish the physical and chemical properties of matter

Physical properties describes a substance without reference to any other substance. A chemical property describes a substance when it combines with another substance. Matter- anything that has mass and occupies space

Understand how to examine glass fractures to determine the direction of impact for a projectile

Radical fracture- A crack in a glass that extends outward like at the spoke of a wheel from the point at which the glass was truck. Concentric fracture- A crack in a glass that forms a rough circle around the point of impact. By analysing the radial and concentric fracture pattern in glass, the forensic scientist can determine the direction of impact by applying the 3R rule: Radial cracks form a Right angle on he Reverse side of the force.

Important Physical Properties

Temperature is a measure of heat intensity, or the hotness or coldness of a substance. Weight is the force with which gravity attracts a body. Mass refers to the amount of matter an object contains independent of gravity. 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.

Glass Fragments

Tempered glass is stronger than normal glass due to rapid heating and cooling. To compare glass fragments, a forensic scientist evaluates two important physical properties: density and refractive index.

Describe the electromagnetic spectrum

The Electromagnetic Spectrum- The entire range of radiation from the most energetic cosmic rays to the least energetic radio waves. Visible Light- Colored light ranging from red to violet in the electromagnetic spectrum. X-ray-The high-energy, short-wavelength from of electromagnetic radiation. Laser- An acronym for light amplification by stimulated emission of radiation; light that has all its weaves pulsating in unison. A photon- A discrete particle of electromagnetic radiation.

Physical Properties of Light

The bending of light waves because of a change in velocity is called refraction. Refractive index is the ratio of the velocity of light in a vacuum to that in the medium under examination. Like density, refractive index is an intensive property and will serve to characterize a substance.

Understand how to use the basic units of the metric system

The metric system has basic units of measurement for length, mass, and volume; they are the meter, gram, and liter, respectively.

Analysing 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). Radial Cracks form first, opposite the side of force Concentric cracks occur afterwards, starting on the same side as the force 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 direction of impact can also be accomplished by applying the 3R Rule: Radial cracks form a Right angle on the Reverse side of the force 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.