UNIT 3: FORENSIC PHYSICS
GLASS EVIDENCE:
Composition of Glass: Glass is primarily made of silica (sand) combined with soda (sodium carbonate) and lime (calcium oxide), which are melted
together to form a solid, transparent material. Glass evidence includes fragments from sources like windows, bottles, or car windows. It's often found at
crime scenes, especially in cases of break-ins, accidents, or assaults. Glass fragments can link a suspect to a location or event
Types of Glass and Characteristics:
1. Soda-Lime Glass: The most common type, used in windows and bottles; it’s inexpensive but not very heat-resistant.
2. Borosilicate Glass: Contains boron; very heat-resistant, used in lab glassware and kitchenware (like Pyrex).
3. Tempered Glass: Treated with heat or chemicals to increase strength; breaks into small, less sharp pieces, making it safer.
4. Lead Glass (Crystal): Contains lead oxide; heavier, with a sparkling appearance, used in fine glassware and decorations.
5. Fiberglass: Made with fine glass fibers; strong and lightweight, often used for insulation and boat materials.
FIBRE GLASS
Soda Lime Glass Borosilicate glass Tempered Glass Lead glass (crystals)
Collection of Glass Evidence
- Use tweezers or gloved hands to carefully pick up glass fragments.
- Place large pieces in a hard, sturdy container to prevent breakage.
,- For small particles, collect with tape or by brushing gently into a container.
- Avoid mixing fragments from different sources.
Packaging of Glass Evidence
- Wrap each fragment separately in paper to prevent cross-contamination.
- Use plastic vials or cardboard boxes for fragile pieces.
- Label all containers with details like date, time, location, case no. evidence no. and collector's name.
MECHANICAL FIT
Analysis of Glass Evidence
- Physical Properties: Check size, shape, color, and thickness to compare fragments.
Matching of Glass Samples:
1. Mechanical Fit: Mechanical fit involves matching broken pieces of glass to see if they fit together
like a puzzle.
Method: Forensic scientists look at the edges, cracks, and patterns on the broken pieces. If two pieces
match perfectly, they likely came from the same object.
Application: This method is helpful when dealing with glass pieces broken from windows, bottles, or
car windshields at a crime scene.
2. Refractive Index (RI) Measurement:
- Definition: The refractive index is a measure of how light bends as it passes through a material.
Each type of glass has a unique RI.
- Method: Scientists use a tool called a refractometer. They place a small glass fragment in a liquid
with a known RI. They adjust the RI of the liquid until the glass fragment "disappears," meaning it
has the same RI as the liquid.
,Both methods are important in forensic science for matching glass fragments to their origin, especially in crimes involving break-ins, hit-and-run
accidents, and other cases where glass is broken.
Analysis by spectroscopic methods:
Spectroscopy: Spectroscopy is the study of how matter interacts with light. When light hits an object, it can be absorbed, reflected, or transmitted.
Different materials absorb or emit light at specific wavelengths, creating a unique “fingerprint” called a spectrum. This fingerprint helps identify the
material's composition.
Common Spectroscopic Methods for Glass Analysis
1. Fourier-Transform Infrared (FTIR) Spectroscopy: FTIR helps detect the types of chemical bonds in the glass by measuring how it absorbs
infrared light. Each chemical bond in glass will absorb light at different wavelengths, creating a unique pattern. FTIR can be used to identify different
types of glass, like window glass or bottle glass.
2. Raman Spectroscopy: Raman spectroscopy involves shining a laser on the glass and measuring the scattered light. It is especially useful because it
does not damage the sample and works well with small glass fragments. Like FTIR, it helps in identifying the chemical composition of glass by
detecting specific molecular vibrations.
3. X-Ray Fluorescence (XRF): XRF is used to determine the elemental composition of glass. When glass is hit by X-rays, it emits secondary X-rays
that reveal which elements are present. This technique helps in comparing glass fragments from a crime scene with glass from a suspect's clothing or
belongings.
4. Inductively Coupled Plasma Mass Spectrometry (ICP-MS): ICP-MS is a powerful technique that measures trace elements in glass at very low
concentrations. This can be useful for determining the exact source of the glass if other methods are inconclusive.
Spectroscopic Methods
i. Non-Destructive: Many spectroscopic techniques don't destroy the sample, which is crucial if only a small amount of evidence is available.
ii. High Sensitivity: These methods can detect even tiny differences in glass composition.
, iii. Quick Identification: Spectroscopy provides fast, reliable results, which is helpful during investigations.
FRACTURE ANALYSIS AND DIRECTION OF IMPACT
Fracture analysis of glass is a valuable technique in forensic science that helps investigators determine
how and where a piece of glass was broken. When glass breaks, it leaves patterns that reveal information
about the force and direction of impact, which can provide clues about the events of a crime.
Types of Fractures in Glass
There are two main types of fractures in glass when it breaks:
1. Radial Fractures: These are cracks that spread outward from the point of impact, like spokes on a
wheel. Radial fractures form on the side of the glass opposite to the impact. For instance, if someone
throws a rock at a window, radial cracks will appear on the inside of the window, opposite to where the
rock hit.
2. Concentric Fractures: These are circular cracks that form around the point of impact, like ripples in a pond. Concentric fractures are usually caused
by continued pressure on the glass after the initial impact. They appear on the same side of the glass as the impact.
Determining the Direction of Impact
Forensic scientists use the patterns of radial and concentric fractures to determine the direction from which the force was applied.
- First Fracture Side: When an object hits glass, the opposite side from the impact is the first to start cracking. This is because glass is weaker under
tension (being pulled apart) than it is under compression (being pushed together).
- Fracture Lines: Radial fractures form first on the opposite side of the impact, while concentric fractures appear on the side where the impact
happened. By examining these fractures, forensic experts can often figure out where the object hit the glass, helping them understand the sequence of
events.
GLASS EVIDENCE:
Composition of Glass: Glass is primarily made of silica (sand) combined with soda (sodium carbonate) and lime (calcium oxide), which are melted
together to form a solid, transparent material. Glass evidence includes fragments from sources like windows, bottles, or car windows. It's often found at
crime scenes, especially in cases of break-ins, accidents, or assaults. Glass fragments can link a suspect to a location or event
Types of Glass and Characteristics:
1. Soda-Lime Glass: The most common type, used in windows and bottles; it’s inexpensive but not very heat-resistant.
2. Borosilicate Glass: Contains boron; very heat-resistant, used in lab glassware and kitchenware (like Pyrex).
3. Tempered Glass: Treated with heat or chemicals to increase strength; breaks into small, less sharp pieces, making it safer.
4. Lead Glass (Crystal): Contains lead oxide; heavier, with a sparkling appearance, used in fine glassware and decorations.
5. Fiberglass: Made with fine glass fibers; strong and lightweight, often used for insulation and boat materials.
FIBRE GLASS
Soda Lime Glass Borosilicate glass Tempered Glass Lead glass (crystals)
Collection of Glass Evidence
- Use tweezers or gloved hands to carefully pick up glass fragments.
- Place large pieces in a hard, sturdy container to prevent breakage.
,- For small particles, collect with tape or by brushing gently into a container.
- Avoid mixing fragments from different sources.
Packaging of Glass Evidence
- Wrap each fragment separately in paper to prevent cross-contamination.
- Use plastic vials or cardboard boxes for fragile pieces.
- Label all containers with details like date, time, location, case no. evidence no. and collector's name.
MECHANICAL FIT
Analysis of Glass Evidence
- Physical Properties: Check size, shape, color, and thickness to compare fragments.
Matching of Glass Samples:
1. Mechanical Fit: Mechanical fit involves matching broken pieces of glass to see if they fit together
like a puzzle.
Method: Forensic scientists look at the edges, cracks, and patterns on the broken pieces. If two pieces
match perfectly, they likely came from the same object.
Application: This method is helpful when dealing with glass pieces broken from windows, bottles, or
car windshields at a crime scene.
2. Refractive Index (RI) Measurement:
- Definition: The refractive index is a measure of how light bends as it passes through a material.
Each type of glass has a unique RI.
- Method: Scientists use a tool called a refractometer. They place a small glass fragment in a liquid
with a known RI. They adjust the RI of the liquid until the glass fragment "disappears," meaning it
has the same RI as the liquid.
,Both methods are important in forensic science for matching glass fragments to their origin, especially in crimes involving break-ins, hit-and-run
accidents, and other cases where glass is broken.
Analysis by spectroscopic methods:
Spectroscopy: Spectroscopy is the study of how matter interacts with light. When light hits an object, it can be absorbed, reflected, or transmitted.
Different materials absorb or emit light at specific wavelengths, creating a unique “fingerprint” called a spectrum. This fingerprint helps identify the
material's composition.
Common Spectroscopic Methods for Glass Analysis
1. Fourier-Transform Infrared (FTIR) Spectroscopy: FTIR helps detect the types of chemical bonds in the glass by measuring how it absorbs
infrared light. Each chemical bond in glass will absorb light at different wavelengths, creating a unique pattern. FTIR can be used to identify different
types of glass, like window glass or bottle glass.
2. Raman Spectroscopy: Raman spectroscopy involves shining a laser on the glass and measuring the scattered light. It is especially useful because it
does not damage the sample and works well with small glass fragments. Like FTIR, it helps in identifying the chemical composition of glass by
detecting specific molecular vibrations.
3. X-Ray Fluorescence (XRF): XRF is used to determine the elemental composition of glass. When glass is hit by X-rays, it emits secondary X-rays
that reveal which elements are present. This technique helps in comparing glass fragments from a crime scene with glass from a suspect's clothing or
belongings.
4. Inductively Coupled Plasma Mass Spectrometry (ICP-MS): ICP-MS is a powerful technique that measures trace elements in glass at very low
concentrations. This can be useful for determining the exact source of the glass if other methods are inconclusive.
Spectroscopic Methods
i. Non-Destructive: Many spectroscopic techniques don't destroy the sample, which is crucial if only a small amount of evidence is available.
ii. High Sensitivity: These methods can detect even tiny differences in glass composition.
, iii. Quick Identification: Spectroscopy provides fast, reliable results, which is helpful during investigations.
FRACTURE ANALYSIS AND DIRECTION OF IMPACT
Fracture analysis of glass is a valuable technique in forensic science that helps investigators determine
how and where a piece of glass was broken. When glass breaks, it leaves patterns that reveal information
about the force and direction of impact, which can provide clues about the events of a crime.
Types of Fractures in Glass
There are two main types of fractures in glass when it breaks:
1. Radial Fractures: These are cracks that spread outward from the point of impact, like spokes on a
wheel. Radial fractures form on the side of the glass opposite to the impact. For instance, if someone
throws a rock at a window, radial cracks will appear on the inside of the window, opposite to where the
rock hit.
2. Concentric Fractures: These are circular cracks that form around the point of impact, like ripples in a pond. Concentric fractures are usually caused
by continued pressure on the glass after the initial impact. They appear on the same side of the glass as the impact.
Determining the Direction of Impact
Forensic scientists use the patterns of radial and concentric fractures to determine the direction from which the force was applied.
- First Fracture Side: When an object hits glass, the opposite side from the impact is the first to start cracking. This is because glass is weaker under
tension (being pulled apart) than it is under compression (being pushed together).
- Fracture Lines: Radial fractures form first on the opposite side of the impact, while concentric fractures appear on the side where the impact
happened. By examining these fractures, forensic experts can often figure out where the object hit the glass, helping them understand the sequence of
events.
