DECEMBER 19, 2012 EQ: HOW IS GLASS ANALYZED TO SOLVE CRIMES? WARM-UP: (Write the questions & answer the questions while watch the following the video.) 1.What are the names of the Central Park Five? 2.Were they innocent of committing any crimes? 3.Why do you think they confessed to raping the victim? 4.What role did the media play in how they presented the story?

Forensic Analysis of Glass:

Glass Links a suspect at the scene of a crime Links a suspect at the scene of a crime Hard, brittle, amorphous (atoms or molecules are arranged in random or disordered positions) substance composed of sand mixed with various metal oxides Hard, brittle, amorphous (atoms or molecules are arranged in random or disordered positions) substance composed of sand mixed with various metal oxides

TEMPERED GLASS Glass to which strength is added by introducing stress through rapid heating and cooling of the glass surfaces Glass to which strength is added by introducing stress through rapid heating and cooling of the glass surfaces When it breaks it does not shatter, but fragments or “dices” into small squares with little splintering When it breaks it does not shatter, but fragments or “dices” into small squares with little splintering Ex. side and rear windows of cars Ex. side and rear windows of cars

LAMINATED GLASS Two sheets of ordinary glass bonded together with a plastic film Two sheets of ordinary glass bonded together with a plastic film Ex. Windshields of U.S. cars Ex. Windshields of U.S. cars

Rolled GlassFloat Glass Float Glass Process

Flotation Method: standard/reference glass is immersed in bromoform and bromobenzene and bromo. is added until the glass floats glass density=liquid density

Refractive Index- When 2 substances have different RI, light passing through them produces a Becke Line A Becke line- a bright halo observed near the border of a particle immersed in a liquid (usually silicon oil) of a different refractive index

Immersion Method glass particles are immersed in a liquid whose refractive index is adjusted until it equals that of the glass particles (match point) The Becke line disappears-medium and fragment have similar refractive indices

JANUARY 9, 2013 EQ: HOW CAN GLASS FRACTURES BE USED TO ANALYZE A CRIME SCENE? WARM-UP: (Write the questions & answers) 1.What is the name of the structure from which hair grows? 2.Name (2) characteristic used to identify synthetic fibers. 3.What type of glass is used on the rear window of cars?

Glass databases using the refractive indices (RI) GLASS DATABASES:

Populations of Glass RIs

Glass bends in response to any force exterted on it’s surface

Direction of Penetration A projectile hole is inevitably wider at the exit side bsapp.com

Direction of Penetration 2. Concentric 1. Radial bsapp.com

How does Glass break? Forensic Fracture Analysis

Radial Begin at a point and radiate outward from point of impact -initial crack is on the side opposite the applied force

Concentric Motion of the projectile through the glass puts tension on the front surface of the glass, causing concentric fractures. -the exit hole helps to determine the direction on impact.

Fractures Concentric Radial bsapp.com

Stress Marks Radial Cracks-the perpendicular edge of stress marks is always found opposite the side where the force was applied bsapp.com

Concentric Fractures: (opposite of radial) The perpendicular end always faces the surface on which the force originated

Direction of Penetration 2. Concentric 1. Radial bsapp.com

3R Rule: Radial cracks form a Right angle on the Reverse side of the force. Radial cracks form a Right angle on the Reverse side of the force.

Counting Bullet Holes Determination of the sequence of bullet holes can be made by noting the radial fractures. Radial fractures caused by the passage of a bullet stop at pre- existing fracture.

Successive Penetrations A fracture always terminates at an existing line of fracture bsapp.com