What is glass?

1
Glass

• What is glass?
• Forces that Fracture Glass

2
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

3
Solid or Liquid? Neither?
Crystalline solid Fluid
Glass Amorphous

• Glass is considered a solid because it is rigid

4
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

5
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

7

• For example, a chip of glass from a broken window
  may fall into a perps 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 materials mass per unit
  volume
• Dmass/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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3
4
1
No refraction beads disappear
2
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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
Air
Ex. Water to air More dense to less dense Light
is bent away from the normal
Water
Angle 1 angle of incidence Angle 2 angle of
refraction
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Which medium is more dense?
Normal
2
Medium 2
Interface
Medium 1
1
Medium 1
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Snells Law

• (n1)(sin angle 1) (n2)(sin angle 2)
• (n1) refractive index of first medium
• (n2) refractive index of second medium

Normal
1
Medium 1
Medium 2
2
Angle 1 2 measured to the normal
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R.I. using submersion method
1 2 3 4
5
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.
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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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Lets try it!Heres 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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Normal
Calculating the refractive index of a liquid
Oil
30
Piece of paper
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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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Place a dot along image line. Draw in line.

• (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

2
Measure angle 2 and apply Snells Law
Solve for refractive index of liquid
1
Laser path
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What it looks like
Normal
47
30
30
To review - Snells 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 came First, because the
right Lines terminate at the Lines of the left one
47
Determine the sequence
B
A