1. 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

6. Appearance – shape, color, thickness Density
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 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.

8. 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

9. 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.

10. 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

11. 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

12. 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 = 200 mL

13. Clean up your area when you are done.
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

14. Part II – Refraction and Patterns

15. Refraction: bending of light as it passes from one medium to another

16. No refraction – beads disappear3 4 1
No refraction – beads disappear 2

17. 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 = or ~1.00

18. Refraction Normal 1 Air Water 2 Angle 1 = angle of incidence
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

19. Which medium is more dense?
Normal 2
Medium 2
Interface
Medium 1
Medium 1 1
Medium 1

20. Angle 1 & 2 measured to the normal
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

21. 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.

22. Glass disappears in vegetable oil
Glass & oil have same R.I.
CSI NY video clip

23. Table of refractive indices
Refractive index video clip

24. Activity: How do we calculate the Refractive Index of a liquid?

25. 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

26. Calculating the refractive index of a liquid
Normal
Calculating the refractive index of a liquid
Oil
30°
Piece of paper

27. Many choices for paper Plain, unlined paper Paper with lines
Polar graph paper (with or without degrees) either unmarked or with lines

28. Solve for refractive index of liquid
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 Snell’s Law
Solve for refractive index of liquid 1
Laser path

29. What it looks like
Normal
47°
30°

30. (n1) (sin 30° ) = (1) (sin 47°) (n1) (.5000) = (1) (.7313)
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

31. 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?

32. Glass fractures

33. 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

34. 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

35. Fracturing glass

36. 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

37. Glass fracture comparison

38. Glass Is considered a slightly elastic solid
Flexes, then breaks to relieve stress at its elastic limit
Teaching tip: Glass under tension is being stretched, whereas glass under compression is being pushed together. When a material is stretched, it is weaker than when it is compressed, and it is the weaker, stretched side that will break frist.

41. 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

42. Path of projectile

43. 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

44. Not to scale Velocity of impact
High velocity (bullet) impact Lower velocity (hammer) impact
Not to scale

45. 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.

46. The left fracture came
First, because the right
Lines terminate at the
Lines of the left one

47. Determine the sequenceB A