Velocity of light in medium Velocity of light in vacuum R.I. = FACTS:

Refractive Index LiquidRIGlassRI Water1.333Vitreous silica1.458 Olive oil1.467Headlight Glycerin1.473Window Castor oil1.82Bottle Clove oil1.543Optical Bromobenzene1.560Quartz Bromoform1.597Lead Cinnamon oil1.619Diamond2.419

For example, at 25°°C the refractive index of water is This means that light travels times faster in a vacuum than it does in water.

Application of Refractive Index to Forensics Place the glass fragment into different liquids of known refractive indexes Place the glass fragment into different liquids of known refractive indexes The glass fragment will seem to disappear when placed in a liquid of the same refractive index The glass fragment will seem to disappear when placed in a liquid of the same refractive index

Amorphous vs. Crystalline Structure Amorphous Structure: GlassCrystalline Structure: Salt (NaCl)

Types: float glass (Annealed) - typical window glass Pyrex – heat resistant Tempered glass – side and rear window of cars, when it breaks - it shatters into small squares with little splintering. Laminated glass - windshields – strong b/c a sheet of plastic is sandwiched btw two glass windows.

Overview of Tempered Glass: Tempered glass is made using special heated furnaces, which heat the annealed glass to a uniform temperature of approximately 700ºC. When the glass exits the furnace, it is rapidly cooled. This rapid cooling puts the glass surface into a state of compression while the center core remains in tension. Mechanical Strength of Tempered Glass = 4 x Mechanical strength of Annealed Glass When tempered glass is broken, all the stored energy is released at once and the glass breaks into small pieces.

Laminated Glass: Car Front Windshields

Overview of Laminated Glass Laminated glass, regard as “Safety glass”, consist of two or more panes of glass with one or more layers of polyvinyl butyral (PVB) sandwiched between them and treated. The glass panes can be basic float glass or tempered or heat strengthened panel. If the glass is broken fragments tend to adhere to the PVB interlayer thereby reducing the risk of injury from falling glass and helping to resist further impact or weather damage.

Comparing glass fragments- Measuring properties that will associate one fragment with another – Density, refractive index, striations, fit, type

Determining Density – Floatation – a known standard glass fragment is immersed into a magic liquid, the composition of the liquid is changed by adding different magic potions to it. This is done until the glass fragment becomes magically suspended in the liquid.

Refractive index – The glass fragments are immersed in another magic liquid (one whose refractive index is known), the liquid is then adjusted until the index reaches that of the glass. When this happens the “ Becke Line ” has completely disappeared.

Becke line – Indication of Refractive Index Notice the halo of light on the inside perimeter of the glass sample. Notice the halo of light on the inside perimeter of the glass sample. When the Becke line is inside the perimeter of the glass fragment, the refractive index of the glass is higher than the refractive index of the surrounding medium. When the Becke line is inside the perimeter of the glass fragment, the refractive index of the glass is higher than the refractive index of the surrounding medium.

Becke line Indication of Refractive Index Notice the halo of light (Becke line) is outside the perimeter of the glass fragment. Notice the halo of light (Becke line) is outside the perimeter of the glass fragment. When the Becke line is outside the perimeter of the glass sample, the refractive index of the medium is higher than the refractive index of the glass. When the Becke line is outside the perimeter of the glass sample, the refractive index of the medium is higher than the refractive index of the glass.

Glass has higher refractive index-note white line inside Glass has lower refractive index-note white line outside Becke Lines

Becke Lines from Glass RI of glass (1.525) > RI of medium (1.6) RI of glass (1.525 < RI of medium (1.34) Becke line on inside Becke line on outside

For example, at 25°°C the refractive index of water is This means that light travels times faster in a vacuum than it does in water.

Snell’s Law N=1.52 The higher the n, the more the light bends N=1.33

frequency of occurrence of refractive index values of approx 2000 flat glass specimens obtained by the FBI

Radial fractures – fracture pattern in which cracks radiate outward Concentric fracture - fracture pattern in which cracks encircle the hole - Typical of a rock, bullet, hammer, etc

How does Glass break? Forensic Fracture Analysis

Determining direction of force Radial edge of glass Radial edge of glass –3 R’s  Radial cracks have Right angles on the Reverse side of applied force on the Reverse side of applied force Concentric edge of glass Concentric edge of glass Direction of force Stress lines on the glass edge of concentric cracks form a right angle on the same side as the force.

Exceptions to 3R rule…. Tempered glass Tempered glass –“dices” without forming ridges Very small windows held tightly in frame Very small windows held tightly in frame –can’t bend or bulge appreciably Windows broken by heat or explosion Windows broken by heat or explosion –no “point of impact” –curved, smooth edges at break points

A new fracture line will always stop when it reaches an existing fracture line Therefore, fracture lines from the first penetration will not end at any other fracture lines