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? Glass is considered a solid because it is rigid Crystalline solid Fluid Glass Amorphous

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. 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 500-300 = 200 mL

13. 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. 3 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 = 1.0003 or ~1.00

18. Normal 1 2 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

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

20. 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 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.

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

23. Refractive index video clip Table of refractive indices

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. 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. 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. 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

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. 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 sequence B A