Published on January 11, 2008
Properties of Matter & Analysis of Glass: Properties of Matter & Analysis of Glass Chapter 4 Physical vs. Chemical Properties: 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. A chemical property describes the behavior of a substance when it reacts or combines with another substance. Measurement System: 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. Slide4: Volume equivalencies in the metric system. The Theory of Light: The 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. 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. The Theory of Light: The 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. 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. Slide7: The electromagnetic spectrum. Dispersion of Light: Dispersion of Light Important Physical Properties: 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. 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. Slide10: The measurement of mass. Important Physical Properties: 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. Important Physical Properties: Important Physical Properties 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 is the ratio of the velocity of light in a vacuum to that in the medium under examination. Light Refraction Diagram: Light Refraction Diagram Light is refracted when it travels obliquely from one medium to another. Important Physical Properties: Important Physical Properties For example, at 25oC 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. Glass Fragments: Glass Fragments 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. Tempered glass is stronger than normal glass due to rapid heating and cooling. Laminated glass found in car windshields has a layer of plastic between two pieces of ordinary window glass. Glass Fragments: 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. To compare glass fragments, a forensic scientist evaluates two important physical properties: density and refractive index. Flotation Method: 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. 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. Refractive Light Method: 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. 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. Immersion Method: Immersion Method The flotation and the immersion methods are best used to determine a glass fragment’s density and refractive index, respectively. The latter 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. Analyzing Cracks: 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. Analyzing Cracks: 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. 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. Glass Fractures Diagram: Glass Fractures Diagram 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. Bullet Holes in Glass: Bullet Holes in Glass Two bullet holes in a piece of glass. The left hole preceded the right hole. Collection of Glass: 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. Collection of Glass: 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.