GLASS
WHAT IS GLASS?
FORCES THAT FRACTURE GLASS
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
SOLID OR LIQUID? NEITHER?
Crystalline solid
Fluid
Glass
Amorphous
Glass is considered a solid because it is rigid
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
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
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
• 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.
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
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.
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
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
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
• 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
PART II – REFRACTION AND PATTERNS
REFRACTION: BENDING OF LIGHT AS
IT PASSES FROM ONE MEDIUM TO
ANOTHER
3
4
1
2
No refraction –
beads disappear
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
REFRACTION
Normal
1
Ex. Air to water
Less dense to more dense
Air
Light is bent toward the normal
Ex. Water to air
Water
More dense to less dense
2
Angle 1 = angle of incidence
Angle 2 = angle of refraction
Light is bent away from the normal
WHICH MEDIUM IS MORE DENSE?
Normal
2
Medium 2
Interface
Medium 1
1
Medium 1
SNELL’S 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
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.
Glass
disappears
in
vegetable
oil
Glass & oil
have same
R.I.
CSI NY video clip
TABLE OF REFRACTIVE
INDICES
Refractive index
video clip
ACTIVITY:
HOW DO WE CALCULATE THE
REFRACTIVE INDEX OF A LIQUID?
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
•
•
•
•
Normal
Calculating the
refractive index
of a liquid
Oil
Piece of paper
30°
MANY CHOICES FOR
PAPER
• Plain, unlined paper
• Paper with lines
• Polar graph paper (with or without degrees) either
unmarked or with lines
Place a dot
along image line.
Draw in line.
2
(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
Measure angle 2 and
apply Snell’s Law
Solve for refractive
index of liquid
1
Laser
path
WHAT IT LOOKS LIKE
Normal
47°
30°
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
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?
GLASS FRACTURES
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
• 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
FRACTURING GLASS
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
GLASS FRACTURE COMPARISON
Glass
• Is considered a slightly elastic solid
• Flexes, then breaks to relieve stress at its elastic
limit
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
PATH OF PROJECTILE
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
VELOCITY OF IMPACT
High velocity (bullet) impact
Lower velocity (hammer) impact
Not to scale
• 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.
The left fracture came
First, because the right
Lines terminate at the
Lines of the left one
DETERMINE THE SEQUENCE
B
A