9 Matter, Light, and Glass Examination

Physical vs. Chemical Properties The forensic scientist must constantly determine those properties that impart distinguishing characteristics to matter, giving it a unique identity. Physical properties such as weight, volume, color, boiling point, and melting point describe a substance without reference to any other substance.

Physical vs. Chemical Properties A chemical property describes the behavior of a substance when it reacts or combines with another substance.

Introduction An element is the simplest substance known and provides the building block from which all matter is composed. Matter is anything that has a mass and occupies space. All of the elements are listed by name and symbol in the periodic table.

Introduction Two or more elements combine to form a compound. An atom is the basic particle of an element and a molecule is the smallest unit of a compound.

Introduction Matter can be classified according to the physical form it takes. Solid-definite shape and volume Liquid-specific volume, takes the shape of its container Gas/vapor-neither a definite shape nor volume

FIGURE 9–1 The periodic table.

Introduction Substances can change from one phase to another without forming a new chemical species, matter is simply being changed from physical state to another. Whenever a situation exists in which a substance can be distinguished by a visible boundary, different phases exist.

Measurement System Scientists throughout the world use the metric system of measurement. The metric system has basic units of measurement for length, mass, and volume; they are the meter, gram, and liter, respectively. The following are common prefixes used in the metric system: deci, centi, milli, micro, nano, kilo, and mega.

Volume equivalencies in the metric system.

Comparison of the metric and English systems of length measurement; 2 Comparison of the metric and English systems of length measurement; 2.54 centimeters 5 1 inch.

Important Physical Properties Temperature is a measure of heat intensity, or the hotness or coldness of a substance. In science, the most commonly used temperature scale is the Celsius scale. This scale is derived by assigning the freezing point of water a value of 0°C and its boiling point a value of 100°C.

Important Physical Properties Weight is the force with which gravity attracts a body. Mass refers to the amount of matter an object contains independent of gravity. The mass of an object is determined by comparison to the known mass of standard objects.

Comparison of the Celsius and Fahrenheit temperature scales.

The measurement of mass.

Density 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.

FIGURE 9–2 A simple procedure for determining the density of a solid is first to measure its mass on a scale and then to measure its volume by noting the volume of water it displaces.

Refractive Index Light waves travel in air at a constant velocity until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend. The bending of light waves because of a change in velocity is called refraction.

Refractive Index Refractive index is the ratio of the velocity of light in a vacuum to that in the medium under examination.

FIGURE 9–3 Light is refracted when it travels obliquely from one medium to another.

Refractive Light Method Crystalline solids have definite geometric forms because of the orderly arrangement of their atoms. These solids refract a beam of light in two different light-ray components. This results in double refraction.

Refractive Light Method Birefringence is the numerical difference between these two refractive indices. Not all solids are crystalline in nature. For example, glass has a random arrangement of atoms to form an amorphous or noncrystalline solid.

Refractive Index For example, at 25°C the refractive index of water is 1.333. This means that light travels 1.333 times faster in a vacuum than it does in water. Like density, refractive index is an intensive property and will serve to characterize a substance.

Theory of Light Two models describe the behavior of light. Light is described as a continuous wave. Light is depicted as a stream of discrete energy particles. When white light passes though a prism, it is dispersed into a continuous spectrum of colors.

Theory of Light Visible light ranges in color from red to violet in the electromagnetic spectrum. Waves are described in terms such as: Wavelength, the distance between two successive crests (or one trough to the next trough). Frequency, the number of crests (or troughs) passing any one given point per unit of time.

FIGURE 9–5 Representation of the dispersion of light by a glass prism.

Theory of Light Frequency and wavelength are inversely proportional to one another. The electromagnetic spectrum is the entire range of radiation energy from the most energetic cosmic rays to the least energetic radio waves. Visible light is only a small part of the electromagnetic spectrum.

Theory of Light As electromagnetic radiation moves through space, its behavior can be described as that of a continuous wave; however, once radiation is absorbed by a substance, it is best described as discrete particles of light known as photons.

FIGURE 9–6 The frequency of the lower wave is twice that of the upper wave.

FIGURE 9–8 Coherent and incoherent radiation.

Glass Composition Glass is a hard, brittle that is composed of silicon oxides (sand) mixed with various metal oxides. 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.

Glass Composition Forensic analysts usually examine “soda-lime” used to manufacture most window and bottle glass.

Glass Composition When molten glass is cooled on a bath of molten tin. This glass is called float glass. Tempered glass is stronger than normal glass due to rapid heating and cooling of the glass surfaces.

Glass Composition Laminated glass found in car windshields has a layer of plastic between two pieces of ordinary window glass.

Glass Fragments For the forensic scientist, the problem of glass comparison is one that depends on the need to find and measure those properties that will associate one glass fragment with another while minimizing or eliminating other sources.

Glass Fragments To compare glass fragments, a forensic scientist evaluates two important physical properties: density and refractive index.

Flotation Method The flotation method is a rather precise and rapid method for comparing glass densities. In the flotation method, a glass particle is immersed in a liquid.

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. 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.

Glass Immersion Method The immersion method is best used to determine a glass fragment’s refractive index. The bending of a light wave because of a change in its speed is called refraction. The ratio of the velocity of light in a vacuum to its speed in any medium determines the refractive index of that medium.

Glass Immersion Method This method involves immersing a glass particle in a liquid medium whose refractive index is varied until it is equal to that of the glass particle. The Becke line is a bright halo near the border of a particle that is immersed in a liquid of a different refractive index.

Glass Immersion Method At this point, known as the match point, the Becke line disappears and minimum contrast between liquid and particle is observed.

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.

FIGURE 9–17 Production of radial and concentric fractures in glass FIGURE 9–17 Production of radial and concentric fractures in glass. (a) Radial cracks are formed first, commencing on the side of the glass opposite to the destructive force. (b) Concentric cracks occur afterward, starting on the same side as the force.

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 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.

Analyzing Cracks 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.

FIGURE 9–19 Two bullet holes in a piece of glass FIGURE 9–19 Two bullet holes in a piece of glass. The left hole preceded the right hole.

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.

Collection of Glass 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.

Collection of Glass 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.