Glass Objective Sheet

Chapter 15: Glass Evidence

Student Learning Objectives

Forensic Science: Fundamentals and Investigations, 2nd ed., Bertino & Bertino, 2015

At the conclusion of this chapter, the student should be able to:

Properties of Glass

_____ 1. List the main ingredients used to produce glass.

_____2. Describe how glass is produced.

_____3. Relate the amorphous atomic arrangement of glass to the ways glass fractures when broken.

_____4. Describe characteristics of each type of glass listed below:

a. Crystal or lead glass

b. Pyrex® or Kimax glass®

c. Soda-lime glass

_____ 5. Explain why soda glass is the most common form of glass.

_____6. State the purpose of adding the following ingredients to glass:

a. lead oxides

b. other metal oxides

_____7. Describe how to produce different colored glass.

_____8. Describe properties of crystal or lead glass that make it desirable in works of art and fine glassware.

_____ 9. Contrast the different thickness of the following types of glass:

a. picture-frame glass

b. window glass

c. door glass

Density of Glass

_____10. State the formula and units for determining density of glass.

_____11. Discuss the various methods used to calculate the density of a small section of glass found at a crime scene.

_____12. Given a piece of broken glass, calculate the density of that glass.

_____13. Compare glass densities of evidence glass and glass found on a suspect.

_____14. Discuss how it is possible to obtain the same density for broken pieces of glass whether you are using a small fragment of glass or a larger section of glass. A different density.

Types of Glass

_____15. Describe the structure of laminated glass and how that construction makes it well suited for front windshield in automobiles.

_____16. Describe the structure of tempered, or safety glass, and how its construction makes it well suited for side and back car windows, glass shower doors and tabletops.

_____17. Describe the structure of bullet-resistant glass and how its construction serves to protect people from injuries.

Refractive Index of Glass

____18. Define refraction.

____19. Describe what happens to light as it moves through two different media that have different refractive indexes.

____20. Describes what happens to the appearance of glass when it is submerged into a

medium with:

a. the same refractive index of the glass

b. a different refractive index from the glass

____21. Determine in what medium a particular piece of glass will "disappear," given a

chart of refractive indexes of different media and the refractive

index of the glass.

____22. Given a glass fragment and a chart of the refractive indices of different

media, describe how to use a submersion test to estimate the glass's refractive index.

____23. Explain how knowing the refractive index of glass evidence would be helpful in solving a crime.

____24. Explain how to calculate the refractive index of glass using the speed of light in a vacuum and the speed of light through a specific medium.

____25. State the relationship between the density of a particular medium and its refractive index.

____26. Define normal lines it pertains to the interface of the surfaces of two different media.

____27. When comparing the refractive index of two different media, explain how to determine the normal line.

____28. On a diagram showing light passing through two different media, label:

angle of incidence
angle of refraction
normal line

____29. Demonstrate the refraction of light using a beam of light passing through two different media using an angle of incidence of 30 degrees.

____30. Using a protractor, measure the angle of refraction from the normal line.

____31. Indicate the direction light bends in reference to the normal line as it moves

a. from a less refractive medium to a more refractive medium

b. from a more refractive medium to a less refractive medium

____32. Write the equation for Snell's Law (Activity 15-4 and IRCD only).

____33. Describe how the refractive index of a substance can be estimated by applying Snell's Law (Activity 15-4 and IRCD only).

Becke Lines

_____34. Discuss the appearance of Becke lines when a small piece of glass is placed in a liquid and is viewed under a microscope.

a. Describe the appearance of Becke lines.

b. On which surface will Becke lines form: the area of higher or lower refractive index?

c. Describe what happens if the glass and the medium in which the glass is immersed have the same refractive index.

d. How are Becke lines used to compare refractive indexes of glass fragments found at a crime scene?

_____ 35. Describe the refractive indexes of two media if the Becke line appears inside the perimeter of the glass fragment or if it appears on the outside of the glass fragment.

Glass Fracture Patterns

_____ 36.Compare and contrast the effects of compressing glass (impact on the glass) with the effect of applying tension on glass(side opposite impact).

_____ 37. Given a diagram of a bullet striking glass, identify where the glass is compressed and where the glass is under tension.

_____ 38. Discuss radial (primary) fractures of glass.

a. What do they look like?

b. When do they form?

c. On which side of the glass do radial fractures form: side of tension (side away from impact) or compression (side of impact)?

_____39. Discuss concentric (secondary)fractures of glass.

a. What do they look like?

b. When do they form?

c. On which side of the glass do concentric fractures form: side of tension (side away from impact) or compression (side of impact)?

____ 40. Prepare a table comparing and contrasting radial and concentric fractures. Include in the table:

a. Shape of the fractures

b. The order in which they form

c. On which side of the glass they form: side of tension (side away from impact) or compression (side of impact)?

_____ 41. Compare the number of concentric rings formed by an object striking glass at a high velocity, such as a bullet, as compared to an object striking glass at a lower velocity, such as a rock.

_____ 42. Discuss heat fractures in glass.

a. How do heat fractures differ from fractures caused by impact?

b. On which side of the glass do heat fractures form: the cooler side or the

warmer side?

c. How could the location of heat fractures be of value in solving crimes?

_____43. Discuss bullet fractures in glass.

a. Compare and contrast the exit and entrance holes of bullets in glass.

b. Compare and contrast the shape of concentric fractures of a bullet to the shape of the concentric fractures formed by a rock.

c. Describe how to estimate the angle of entry of a bullet based on the shape of the exit hole in the glass.

_____44. Describe how the sequence of impacts can be determined using glass fracture patterns.

_____45. Given glass with two or more impacts and fractures, determine the sequence of the impacts.

Forensic Glass Evidence

_____46. Cite evidence to support the claim that glass evidence is considered class evidence.

_____47. Describe backscatter as a form of evidence. Include in your answer:

a. Why backscatter is often overlooked.

b. Where would backscatter evidence be located: on the impact side or the side away from the impact?

_____48. List examples of trace evidence found on glass.

_____49. Describe how any trace evidence on glass should be documented and collected.

_____50. Discuss how to properly document glass evidence. Include in your answer:

a. When photos should be taken of the broken glass

b. What should be included in the photos

c. Describe the forensic importance of documenting

i. location of the glass fragments

ii. the location of the glass fragments in reference to other glass

iii. which side of the glass was from the inside of a structure as compared to which side was form the outside of the structure.

iv. presence of any other evidence on the glass

_____51. Summarize how to collect glass evidence. Include in your answer:

a. Should large or small pieces be collected first?

b. How to maintain proper chain of custody

_____52. Given evidence glass and glass found on a suspect, describe how you could compare the two glass samples based on

a. color

b. thickness

c. density

d. refractive index

e. type of glass

_____53. Describe the role of technology in forensic glass analysis. Include in your answer:

a. scanning microscopy

b. X-ray fluorescence (XRF)