 A hard substance made when heat is applied to sand, lime and metal oxides (usually sodium, calcium, magnesium, and aluminum)  Silicon dioxide (SiO 2 ), also called silica, is the primary ingredient in glass  Sodium oxide (Na 2 O) reduces the melting point of silica  Calcium oxide (CaO) prevents the glass from being soluble in water

 Leaded Glass: also called crystal, substitutes lead oxide (PbO) for calcium oxide and is used to make fine glassware and decorative art glass  Colored Glass: created by adding certain metal oxides NiO: produces yellow and purple glass CoO: produces purple-blue glass SeO 2 : produces red glass

 Tempered Glass: made by rapid heating and cooling of the glass; designed to break into small pieces that do not have sharp edges; it is commonly used in side and rear windows in cars  Safety Glass: made by sandwiching a layer of plastic between two pieces of window glass; this type of glass is break resistant and is used in car windshields

 Density: each type of glass has a density that is specific to that glass  Color  Refractive Index

 Refraction is the change in the direction of light as it speeds up or slows down when moving from one medium into another; the direction and amount the light bends varies with the densities of the two mediums  Refractive index: calculated by dividing the speed of light in a vacuum (300,000 km/s) by the speed of light through that particular substance

 When light travels through a vacuum, it does not slow  When light travels through any other medium, the particles in that medium slow the light down  As the density of the medium increases, the speed of light passing through that material decreases

 Normal: a line perpendicular to the surface where the two different mediums meet  Incident Ray: the incoming beam of light passing through the first medium  Refracted Ray: the beam of light passing through the second medium

 Angle of Incidence: the angle the incident ray in medium 1 forms with the normal  Angle of Refraction: the angle the refracted ray in medium 2 forms with the normal

 Describes the behavior of light as it travels from one medium into a different medium  Written as: n 1 (sine angle 1) = n 2 (sine angle 2) Where n 1 is the refractive index of medium 1 n 2 is the refractive index of medium 2 angle 1 is the angle of incidence angle 2 is the angle of refraction

 Submersion method: involves placing the glass fragment into different liquids of known refractive indexes; the glass fragment will seem to disappear when placed in the liquid with the same refractive index

 Caused when a bullet or other object goes through glass  As the bullet passes through the glass, the glass bends and then snaps back  The glass can blow back up to 18 feet

 Round Hole: bullet entered the glass at a right angle  Elongated Hole: as the angle at which the bullet enters the glass increases, the hole becomes more elongated  The exit hole will be larger than the entrance hole; this helps determine from which direction the bullet entered the glass

 When a bullet goes through glass, it usually will not break, but it will crack  The first cracks are in a straight line that extend from the point of impact and form on the opposite side of the point of impact

 Concentric or circular cracks form next around the point of impact and form on the same side of impact  When more than one bullet goes through the glass, the first bullet causes long, thin, uninterrupted cracks

 Subsequent bullets will cause cracks, but these cracks will end when they meet the cracks caused by the previous bullets  By looking at the pattern of the cracks, it is possible to determine the order in which the bullets entered the glass

 High heat produces wavy fracture lines  The glass breaks toward the region of higher temperature  There should be no radial or concentric circle fracture patterns