Forensic Sci Exam 2
Chapter 8 – The Microscope
a. Optical Principles
Optical principles refer to how light interacts with lenses to magnify objects.
Light bends (refracts) when passing through different mediums due to differences in refractive index.
The numerical aperture (NA) of a lens determines its ability to resolve fine details.
Resolution: The ability of a microscope to distinguish two close objects as separate.
b. Compound Microscope
i. Parts of the Compound Microscope
Eyepiece (Ocular Lens): The lens you look through, typically 10x magnification.
Objective Lenses: Typically range from 4x to 100x magnification.
Stage: Holds the specimen slide in place.
Coarse and Fine Focus Knobs: Used to adjust the focus.
Light Source: Provides illumination for viewing the specimen.
ii. Calculating Magnifying Power
Total Magnification = Eyepiece Magnification × Objective Lens Magnification
Example: 10x eyepiece with 40x objective lens = 400x total magnification.
iii. How Field of View Changes with Magnifying Power
As magnification increases, the field of view decreases.
Example: A 4x lens has a wider field of view than a 40x lens.
c. Comparison Microscope
i. Used for Examining What?
Allows side-by-side comparison of specimens under the same magnification.
Used in forensic analysis for ballistics, tool marks, hair, fibers, and paint samples.
d. Stereoscopic Microscope
i. Used for Examining What Type of Evidence?
Provides a three-dimensional view of specimens.
Used for examining large, opaque, or irregularly shaped objects, such as:
Paint chips
Tool marks
Counterfeit currency
Bullets and cartridge cases
e. Scanning Electron Microscope (SEM)
i. Generally How It Works
Uses an electron beam to scan a specimen's surface, producing highly detailed images.
Provides high magnification (up to 100,000x) and depth of field.
Used for forensic analysis of gunshot residue, fibers, and tool marks.
Chapter 10 – Light & Glass Examination
a. Light
i. Refraction & Refractive Index
Refraction: The bending of light when it passes from one medium to another.
Refractive Index (RI): A measure of how much light slows when entering a material.
Example: Glass RI ~1.5
ii. Birefringence
Occurs when a material splits light into two beams traveling at different speeds.
Seen in materials like fibers and minerals.
iii. Dispersion
The separation of light into its component colors (e.g., a prism creating a rainbow).
iv. Visible Light
The portion of the electromagnetic spectrum visible to the human eye (wavelengths ~400-700 nm).
b. Glass
i. Chemical Composition
Made primarily of silicon dioxide (SiO2) with additives like sodium carbonate and calcium oxide.
ii. Properties
Hardness, brittleness, transparency, density, and refractive index.
iii. Manufacturing Processes
Float Glass: Used in windows, made by floating molten glass on tin.
Tempered Glass: Heat-treated for strength; breaks into small pieces.
Laminated Glass: Used in windshields, contains a plastic layer between glass sheets.
iv. Individual vs. Class Characteristics
Class Characteristics: Common to a group (e.g., glass type, thickness, color).
Individual Characteristics: Unique to a specific piece (e.g., fracture patterns, striations).
v. Density & Refractive Index
Density can help identify glass type.
Refractive index can distinguish different glass samples.
vi. Trace Elemental Analysis
Detects small amounts of elements in glass using techniques like ICP-MS.
vii. Glass Fractures
Radial vs. Concentric Fractures
Radial: Form first, radiate outward from impact.
Concentric: Form later, encircle impact.
Stress Marks: Indicate direction of force.
Sequence of Impact: Helps determine order of bullet or object strikes.
viii. Collection & Preservation of Glass Evidence
Collect fragments carefully, place in separate containers.
Document fracture patterns and impact points.
Chapter 11 – Hair & Fibers
a. Hair
i. Morphology
Hair is composed of keratin and consists of cuticle, cortex, and medulla.
ii. Hair Shaft Structure
Cuticle: Outer layer with overlapping scales.
Cortex: Middle layer, contains pigment granules.
Medulla: Inner core, may be continuous, fragmented, or absent.
iii. Roots
Root shape helps determine whether hair was forcibly removed or naturally shed.
iv. Characteristics of Hair from Different Parts of the Body
Head Hair: Uniform diameter, pigment distribution even.
Pubic Hair: Coarse, curly, irregular diameter.
Facial Hair: Coarse with blunt tips.
v. Examination & Analysis of Hair
Microscopic analysis: Identifies species, body area, and treatment (e.g., dyeing).
DNA analysis: Extracted from root for individual identification.
vi. Questions Concerning Hair
Can determine race, species, and possible source, but not conclusive evidence of identity without DNA.
b. Fibers
i. Natural Fibers
Types:
Animal: Wool, silk
Plant: Cotton, flax
Examination: Microscopy for shape, diameter, and surface characteristics.
ii. Manufactured Fibers
Regenerated vs. Synthetic
Regenerated: Made from natural sources (e.g., rayon from cellulose).
Synthetic: Made from petroleum-based polymers (e.g., nylon, polyester).
Polymers: Long chains of repeating molecular units forming synthetic fibers.
iii. Examination & Analysis
Microscopic Analysis:
Color & Diameter: Differentiates fiber types.
Striations: Surface markings.
Delustering Particles: Reduce fiber shine.
Cross-Section: Distinctive shapes aid identification.
Dye Composition: Analyzed using chromatography.
Chemical Composition: Determines polymer type.
Birefringence: Optical property used to identify fibers.