MR

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

  1. Radial vs. Concentric Fractures

    • Radial: Form first, radiate outward from impact.

    • Concentric: Form later, encircle impact.

  2. Stress Marks: Indicate direction of force.

  3. 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

  1. Cuticle: Outer layer with overlapping scales.

  2. Cortex: Middle layer, contains pigment granules.

  3. 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

  1. Types:

    • Animal: Wool, silk

    • Plant: Cotton, flax

  2. Examination: Microscopy for shape, diameter, and surface characteristics.

ii. Manufactured Fibers

  1. Regenerated vs. Synthetic

    • Regenerated: Made from natural sources (e.g., rayon from cellulose).

    • Synthetic: Made from petroleum-based polymers (e.g., nylon, polyester).

  2. Polymers: Long chains of repeating molecular units forming synthetic fibers.

iii. Examination & Analysis

  1. Microscopic Analysis:

    • Color & Diameter: Differentiates fiber types.

    • Striations: Surface markings.

    • Delustering Particles: Reduce fiber shine.

    • Cross-Section: Distinctive shapes aid identification.

  2. Dye Composition: Analyzed using chromatography.

  3. Chemical Composition: Determines polymer type.

  4. Birefringence: Optical property used to identify fibers.