1. Physical Properties: Glass and Soil

2. Physical property: describes the behavior of a substance without having to alter the substance’s composition through a chemical reaction
Chemical property: describes the behavior of a substance when it reacts or combines with another substance
 Weight: a property of matter that depends on the mass of a substance and the effects of gravity on that mass
 Mass: a constant property of matter that reflects the amount of material present
Density: a physical property of matter that is equivalent to the mass per unit volume of a substance

3. Refraction: the bending of a light wave as it passes from one medium to another
 Refractive index: the ratio of the speed of light in a vacuum to its speed in a given substance
 Crystalline solid: a solid in which the atoms have a regular arrangement
Amorphous solid: a solid in which the atoms or molecules are arranged in random or disordered positions. There is no regular order.
 Birefringence: a difference in the two indices of refraction exhibited by most crystalline materials

4. Dispersion: the separation of light into its component wavelengths
Becke line: a bright halo that is observed near the border of a particle immersed in a liquid of different refractive index
 Radial fracture: a crack in a glass that extends outward like the spoke of a wheel from the point at which the glass was struck
 Concentric fracture: a crack in a glass that forms a rough circle around the point of impact
Density-gradient tube: a glass tube filled from bottom to top with liquids of successively lighter densities

5. Physical Evidence: Glass and Soil Analysis

6. Properties of Matter Chemical Properties Physical Properties
a characteristic of a substance that describes the way the substance undergoes or resists change to form a new substance
Physical Properties
a characteristic of a substance that can be observed without changing the substance into another substance

7. Physical Properties Extensive (extrinsic/extensive) Properties
depend on the amount of sample
volume, mass
Intensive (intrinsic/intensive)Properties
do not depend on the amount of sample
melting point, density

8. Metric System
In 1791, the French Academy of Science devised the simple system of measurement known as the metric system.
The metric system has basic units for length, mass, and volume.
These units are the meter, gram, and liter.
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Volume can be defined in terms of length
One liter is the volume of a 10 cm x 10 cm x 10 cm cube
– 1 liter = 1,000 cubic centimeters (cc)
– 1 liter is also 1,000 mL
– Therefore, 1 mL =1 cc
These terms are used interchangeably
by scientists.

9. Physical Properties
The temperature at which a substance melts or boils will help identify a substance.
What is temperature?
Temperature is the amount of heat in an object.
Measured in Fahrenheit ,the values are
32 °F and 212 °F for water
Measured in Celsius, the values are
0 °C and 100 °C for water

10. Weight and Mass
Weight is the force with which gravity attracts a body.
Mass refers to the amount of matter an object contains.
The mass of an object is determined by comparing it against the known mass of standard objects.

11. Density An important physical property of matter Intensive property
Defined as mass per unit volume.
Typically measured in g/mL or g/cm3
Varies by temperature

12. Electromagnetic radiation (light):
Composed of waves that transport energy as photons
Wavelength and Frequency are inversely proportional
Light travels at 3 x 108 m/s in a vacuum
speed of light = wavelength x frequency
(c = ln)

13. Refraction Dispersion
Light Facts
Light travels more slowly in media
The change of wavelength at the surface between different media causes light passing to be bent called refraction
Waves of different frequencies bend at different angles; the results being that the light is dispersed
Refraction Dispersion

14. Refraction
The bending that occurs when a light wave passes at an angle from one medium to another (ex. air to glass)
bending occurs because the velocity of the wave decreases
The Refractive Index (RI) is a highly distinctive property of glass and is useful for evidential value.

15. Snell’s Law: the simple formula used to calculate the refraction of light when travelling between two media of differing refractive index. 
n1 = refractive index of 1st
medium
n2 = refractive index of 2nd
Ө1 = angle of incidence
between incoming ray
and normal line
Ө2 = angle of incidence
between outgoing ray and
normal line

16. The refractive index of a substance is equal to the speed of light in space divided by the speed of light in that particular substance.
Substance State Refractive Index
Air Gas
Ice Solid 1.31
Water Liquid 1.33
Ethyl Alcohol Liquid 1.36
Quartz Solid 1.54
Salt Solid 1.54
Tourmaline Solid 1.62
Garnet Solid
Cubic Zirconium Solid
Diamond Solid 2.41
Almost all refractive indices are determined using the wavelength of 589 nm (predominant wavelength emitted by sodium light)

17. Birefringence The difference between the two indices of refraction
Use in identifying crystals
for example: calcite crystals :
have RI values of & 1.658
birefringence = 0.172

18. Glass
The Basics

19. What is Glass? One of the oldest of all manufactured materials
A simple fusion of sand (SiO2) with a variety of other compounds
An amorphous solid (network of atoms which lacks the repeated, orderly arrangement found in crystalline substances)
Also known as a “super cooled liquid”

20. Physical Properties At ordinary temp.
internal structure resembles a fluid
random molecular orientation
external structure displays the hardness & rigidity of of a solid
Does not show a distinct melting point
on heating gradually softens
on cooling gradually thickens

21. Structure of Glass

22. Comparing Glass Fragments
Composed of silicon oxides mixed with other oxides
Soda-lime glass
(float glass)
Soda (NaCO3) and Lime (CaO)
Windows, bottles
Pyrex Borosilicates
contain Boron oxide, Can withstand HIGH heats
Test tubes
Headlights
Leaded Glass (crystal)
contain Lead oxides
wine glasses,
fine dining, figurines
Tempered Glass
Rapid heating and cooling does not shatter
Shower doors,
Side + rear car windows
Laminated Glass
Plastic or Glass and glues are sandwich
windshields

23. Tempered glass: stronger than ordinary window glass by
introducing stress through rapid heating and cooling of the glass
surfaces.
When tempered glass breaks, it does not shatter but fragments or dices into small squares with little splintering.
Commonly used in side and rear windows of automobiles.
Laminated glass: made by sandwiching one layer of plastic
between two layers of glass.
Found in car windshields.

24. Plate Glass

25. Tempered Glass

26. Laminated glass

27. Glass as Forensic Evidence
Physical properties can be used to place glass into a class
Density
Refractive Index (RI)
Elemental composition
Individualization can’t be determined from these properties alone but only if fragments can be pieced together

28. Flotation Test Based on density comparison
A control glass chip (with a known density)
immersed in a liquid (often a mixture)
composition is altered until the chip remains suspended
The crime object (glass of unknown density) is immersed in the liquid mixture
Remains suspended: the liquid, control & unknown have same density
Sinks: the unknown is more dense than control
Floats: the unknown is more dense than control

29. Refractive Index By Immersion
Several methods
Involves using different liquids with various refractive indices and immersing glass pieces to determine a “match”
Entails finding the temperature at which a glass particle and a liquid have identical refractive indices
refractive index of glass is relatively unaffected by changing temp
Reason why the eye is unable to distinguish between the solid-liquid boundary when RI are equivalent

30. Becke Line
A bright halo that is observed near the border of a particle immersed in a liquid of a different refractive index
When Becke line and glass disappears, index of refraction has been reached
Glass has higher refractive index
Becke line seen inside
Glass has lower refractive index
Becke line seen outside

31. GRIM Glass Refractive Index Measurement
Instrument used for measuring refractive index of glass fragments

32. Refractive Index By Immersion
Hot stage microscope used
Glass is immersed in a liquid with a higher RI than the glass.
Temperature is raised at rate of 0.2 oC/min until the Becke line disappears. (Rate of change of RI in liquid is known)
The point where the Becke line disappears:
RI of the sample = the RI of the liquid

33. Elemental Analysis
Trace elements enter glass via impurities in the raw materials or during manufacturing
Comparison of elemental analysis of crime glass versus reference glass can offer valuable information
if ranges of elements overlap for every elementthey are indistinguishable
if ranges of one or more elements are different
samples are distinguishable

34. Glass Fracture Examination
Occurs when the limit of glass’s elasticity is reached
Gives information relating to the force and direction of an impact
Two types of fracture patterns:
Radial fractures – cracks radiate outward and encircle hole like spokes of a wheel
Concentric fractures – circular lines form a rough circle around point of impact

35. Glass Fractures Sequence:
Radial cracks are formed on opposite surface
Continued force on surface causes concentric cracks on surface side of the force.

36. Impact causes a pane of glass to bulge
Side opposite the impact will bend more & rupture first
Radial cracks are rapidly propagated in short segments from the point of impact

37. Glass Analysis
Ridges on radial cracks can be used to determine on which side impact occurred
Stress marks left on broken edges of glass are perpendicular to one side and curve (run almost parallel) to the other side

38. Three “R” Rule
Ridges (on radial cracks) are at Right angles to the Rear (side opposite the impact)

39. Exceptions to the Three “R” Rule
tempered glass
“dices” without forming ridges
very small windows held tightly in frame
can’t bend or bulge appreciably
windows broken by heat or explosion
no “point of impact”

41. Fractures Caused by Projectiles
Leaves a round, crater shaped hole surrounded by a nearly symmetrical pattern of radial and concentric cracks
Hole is wider at exit side providing a means of determining direction of impact

42. Fractures Caused by Projectiles

43. Bullet Analysis
If a window is broken by a bullet, it is possible to determine the bullet's direction by noting the side of the cone-shaped hole left by the bullet. The small opening is on the entrance side and the large opening is on the exit side.
A determination of the sequence of bullet holes can be made by noting the radial fractures. Radial fractures caused by the passage of a bullet will stop at any pre-existing fracture.