what is glass? forces that fracture glass glass. historically obsidian (volcanic glass) use as...
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WHAT IS GLASS?FORCES THAT FRACTURE GLASS
GLASS
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HISTORICALLY
• Obsidian (volcanic glass) use as cutting tool
• 2500 B.C. glass beads in Egypt• 1st Century B.C. glass blowing• 1291 Murano glass, Venice• Industrial Revolution – mass production of
glass
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SOLID OR LIQUID? NEITHER?
Glass is considered a solid because it is rigid
Crystalline solid Fluid Glass
Amorphous
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WHAT IS GLASS?
• One of the oldest of all manufactured materials
• Hard, amorphous solid• Without shape, particles are arranged
randomly instead of in a definite pattern• A simple fusion of sand, soda, and lime
produces a transparent solid when cooled
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WHY STUDY GLASS?
• Glass has stable, physical properties which can be measured
• May link a suspect to a crime scene• Can determine a sequence of events
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WHAT PROPERTIES CAN BE USED TO DISTINGUISH
BETWEEN OR MATCH GLASS SAMPLES?
• Appearance – shape, color, thickness• Density• Refractive Index (and Becke lines) • Fracture patterns• Chemical analysis
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• For example, a chip of glass from a broken window may fall into a perp’s trouser cuff or shoes.
• A forensic scientist can identify the chips as part of the broken window.
• Similarly, parts of a broken headlight found at the scene of a hit and run can be used to identify the suspected vehicle.
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SAFETY GLASS
• Broken glass can be sharp and dangerous
• car manufacturers use tempered and safety glass in vehicles.
• Tempered glass is made strong by a rapid heating and cooling process that introduces stress to the glass surface
• When tempered glass breaks, it fragments into small squares that do not have sharp edges
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LAMINATED GLASS
• Windshields are made of laminated or safety glass.
• This type of glass is strong and break resistant because it is made by sandwiching a layer of plastic between two ordinary pieces of window glass.
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DIFFERENT DENSITIES FOR DIFFERENT GLASS
• Forensic scientists use physical properties of glass to associate one type of glass fragment with another.
• One of these properties is density• Density refers to a material’s mass per unit volume
• D=mass/volume
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DENSITY
• Density of a substance remains constant, no matter what the size of the substance
• 3 steps to determine density:• 1. weigh the sample to find its mass• 2. determine the volume of the sample• 3. Divide the mass of the sample by its
volume
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HOW TO FIND VOLUME USING WATER DISPLACEMENT METHOD
• Initial volume of water in beaker = 300 mL
• Add a rock• New volume= 500 mL• Volume of rock 500-300 = 200 mL
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• Now it is your turn to calculate the densities of various types of glass using the water displacement method
• You will need• Glass density handout• Glass samples – Be careful!!• Graduated cylinder • Water• Scale• Calculator
• Clean up your area when you are done.• Turn in your completed handout at the end of the
period
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PART II – REFRACTION AND PATTERNS
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REFRACTION: BENDING OF LIGHT AS IT PASSES FROM ONE MEDIUM TO
ANOTHER
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No refraction –
beads disappear
1
2
34
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WHAT IS REFRACTIVE INDEX?
• Comparison of speed of light in vacuum
speed of light in material
• R.I. in vacuum = 1• R.I. in air = 1.0003 or ~1.00
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REFRACTION Ex. Air to water
Less dense to more dense
Light is bent toward the normal
Ex. Water to air
More dense to less dense
Light is bent away from the normal
Water
Air
Normal
1
2
Angle 1 = angle of incidence
Angle 2 = angle of refraction
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WHICH MEDIUM IS MORE DENSE?
Medium 1
Normal
2
1
Medium 2
Medium 1
Interface
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SNELL’S LAW (n1)(sin angle 1) = (n2)(sin angle 2)
(n1) = refractive index of first medium
(n2) = refractive index of second medium
Angle 1 & 2 measured to the normal
Normal
1
2
Medium 1
Medium 2
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R.I. USING SUBMERSION METHOD
Glass will seem to disappear when submerged in a liquid with the same refractive index
Notice - Glass disappears in test tube 4. The refractive
index of the glass and the liquid are the same.
1 2 3 4 5
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Glass
disappears in
vegetable oil
Glass & oil
have same
R.I.CSI NY video clip
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TABLE OF REFRACTIVE INDICES
Refractive index video clip
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ACTIVITY: HOW DO WE CALCULATE THE
REFRACTIVE INDEX OF A LIQUID?
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LET’S TRY IT!HERE’S THE PLAN.
• Draw two perpendicular lines• Draw a third line 30° from the normal• Position your liquid-filled plastic dish• Using the laser pointer, determine the angle of
refraction• Calculate the refractive index of the liquid
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30°
Calculating the refractive index of a liquid
Piece of paper
Normal
Oil
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MANY CHOICES FOR PAPER
• Plain, unlined paper• Paper with lines• Polar graph paper (with or without degrees)
either unmarked or with lines
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Laser path
Place a dot
along image line.
Draw in line.
Measure angle 2 and apply Snell’s Law
Solve for refractive index of liquid
(n1)(sin first angle) = (n2)(sin second angle)
• (n1) = refractive index of first medium
• (n2) = refractive index of second medium
• Angle 1 & 2 measured to normal
1
2
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WHAT IT LOOKS LIKE
Normal
30°
47°
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TO REVIEW - SNELL’S LAW
Medium 1(liquid) Medium 2 (air)(n1)(sin first angle) = (n2)(sin second angle)
(n1) (sin 30° ) = (1) (sin 47°)
(n1) (.5000) = (1) (.7313)
(n1) = .7313/.5000 = 1.46
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TIME TO WORK (AGAIN)!
• Obtain liquid, hemispherical dish, laser pointer, two pins, paper and a partner
• Set up dish as directed• Determine the angle of incidence and
refraction• Calculate the refractive index of the liquid• What steps should be taken to ensure
reasonable results?
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GLASS FRACTURES
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GLASS CAN PROVIDE VALUABLE EVIDENCE ABOUT A CRIME
•Comparisons possible with broken or fractured glass include: •physical match•probability of common origin•direction of impact•Sequence of impact
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• The penetration of glass by a high speed projectile, such as a bullet, can leave evidence as to the direction of impact.
• If there is more than one hole in glass from flying projectiles, the sequence of their impact can be determined
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FRACTURING GLASS
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STRAIGHT AND CIRCULAR LINES
• When glass is penetrated by a projectile, it fractures in two ways
• Radially• Extends from the point of impact• Outward lines found opposite side of impact
• Concentrically• Circular line of broken glass around the point of
impact, on the same side
When a high speed projectile hits glass, it bends the glass as far as possible, then breaks it
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GLASS FRACTURE COMPARISON
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Glass• Is considered a slightly elastic solid• Flexes, then breaks to relieve stress at its elastic limit
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BULLET HOLES• When a high-speed projectile, such as a bullet,
penetrates glass, it leaves an exit hole that is larger than its entrance hole.
• This helps to determine the direction of impact.
• The hole produced is often crater-shaped, and surrounded by concentric and radial fractures.
• A piece of glass may be penetrated by more than one projectile
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PATH OF PROJECTILE
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HIGH OR LOW VELOCITY PROJECTILES
•An impression or hole size in glass can determine whether it was a high or low velocity object
•Check area for evidence – stone v. bullet
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VELOCITY OF IMPACT
High velocity (bullet) impact Lower velocity (hammer) impact
Not to scale
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• It is possible to determine the order in which the penetrations occurred by examining the fracture lines.
• 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.
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The left fracture cameFirst, because the rightLines terminate at theLines of the left one
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DETERMINE THE SEQUENCE
A
B