Characteristics of Glass Glass is defined as a rigid, transparent, amorphous solid formed by continuous cooling from the liquid state.  The microstructure of glass is similar to a liquid in that both lack long-range repeatable order, and because of this similarity, a melting point for glass is difficult to define.  Instead, glasses are characterized by a glass-transition temperature (Tg) which denotes a distinct change in physical properties.  At temperatures below Tg, a glass is hard and rigid, but above Tg it is soft and rubbery.  Silica (SiO2) makes up the bulk of most glasses encounted in everyday life.  However, the overall composition may vary depending on the desired properties.
	Index of refraction  Index of refraction is the ratio of the speed of light in a vacuum to that in a medium.  Since the speed of light in any medium is always slower than when in a vacuum, the refractive index is always greater than one.                n =speed of light in vacuum                             speed of light in medium The reflection and refraction of light is a result of the difference in index of refraction of various media .  When light travels through a medium with a different refractive index, the speed of light changes. Because the speed changes, so does its direction. Light is refracted toward the perpendicular in passing from a medium of lower refractive index to one of higher index. This is defined as Snell's Law.  Snell's Law                    n 1sinq1= n 2sinq2 where n is the index of refraction of the medium and q is the angle of incidence of the light ray. The reflection and refraction of light are two properties that can be exploited analytically to compare types of glass. The Becke line method, comparison of relief, and oblique illumination are three techniques which use the refractive properties of glass.  These routine methods can be used to compare glass samples.  The links on the left will take the viewer to three pages dedicated to each of these methods.
	Forensic Analysis of Glass In forensic investigations, glass is important because it can be found in many everyday products such as bottles, windows, insulation, eye glass lenses, lightbulbs, and mirrors.  Shattered glass from broken bottles, window panes, car windshields, and headlights is commonly found at  crime scenes. The ease of transference of glass particles from the scene to the perpetrator make them particularly valuable in linking suspects to crime scenes.
	Materials and Methods All glass samples were part of the Cargile Standards Reference Set N-1 and were mounted in Permount (n=1.525).  Samples were examined with a Leitz compound microscope. Photos were prepared with a Leica CL camera using 35mm Fuji ISO 800 film.  Specific microscope and camera settings accompany each photo.
	This project was completed by Rebekah Gundry, Kelly Sloan, Isaac Rockoff under the direction of Walter Rowe, PhD.  All comments and suggestions should be directed to Walter Rowe, PhD or the Forensic Science Department