Forensic Science

Matter, Light & Glass Examination

Different Glass Will Bend Light Differently

Light bends (or refracts) when passing through different types of glass because each type has a unique refractive index. The amount of bending depends on the glass’s density and composition.

• Example: Crown glass (used in windows) has a lower refractive index than flint glass (used in optical lenses), causing light to bend less.

Light as Wave vs Energy

Wave Properties of Light

Light behaves as a wave, meaning it has characteristics such as wavelength and frequency.

• Wavelength (λ): The distance between two successive wave crests or troughs. It determines the type of electromagnetic radiation (e.g., radio waves, visible light, X-rays).

  • Example: Red light has a longer wavelength (~700 nm), while blue light has a shorter wavelength (~450 nm).

• Frequency (f): The number of waves that pass a given point per second.

  • Formula: f = c / λ, where c is the speed of light (≈ 3.00 × 10⁸ m/s).

  • Example: Ultraviolet (UV) light has a higher frequency than visible light, which is why it can damage DNA and cause sunburn.

The Electromagnetic Spectrum

The electromagnetic spectrum categorizes different types of light based on wavelength and frequency.

• Example in Forensics: UV and IR light are used to analyze blood spatter, fingerprints, and document forgeries.

Laser – Light Amplification by Stimulated Emission of Radiation

A laser produces coherent, monochromatic light, meaning all waves are in sync and of a single wavelength.

• Forensic Use:

  • Crime Scene Investigation: Lasers help detect fingerprints, bloodstains, and gunshot residues using fluorescence.

  • Glass Analysis: Laser beams can measure refractive index by analyzing how light bends through a sample.

Light as a Particle – Photon

A photon is a packet of energy that light carries. It behaves as both a wave and a particle.

• Example: When UV light hits a material, it can knock electrons loose (the photoelectric effect), used in forensic spectrometers to analyze chemical compositions.

Refractive Index

The refractive index (n) is a measure of how much light slows down and bends when entering a material.

• Formula: n = c / v, where v is the speed of light in the material.

Refraction

Refraction occurs when light changes speed and direction as it enters a different medium.

• Example: A straw appears bent when placed in water because light slows down when entering water from air.

Determining and Comparing Refractive Index – Immersion Method

A forensic technique where glass fragments are placed in liquids of known refractive indices to find a match.

• Becke Line Test: Under a microscope, a bright halo (Becke line) appears around the glass fragment and disappears when the liquid's refractive index matches the glass.

Dispersion

Dispersion is the separation of light into its component colors due to varying refractive indices for different wavelengths.

• Example: A prism disperses white light into a rainbow.

• Forensic Application: Helps determine glass type based on how it splits light.

Visible Light

The part of the electromagnetic spectrum humans can see (400-700 nm). Different materials absorb and reflect different wavelengths, determining their color.

• Example: Forensic scientists use colored filters and alternate light sources to enhance trace evidence like bodily fluids and fibers.

Forensic Analysis of Glass

Composition of Glass

Glass is made from silica (SiO₂), sodium carbonate (Na₂CO₃), and calcium oxide (CaO).

• Properties Change Based on Chemical Composition:

  • Tempered Glass: Stronger due to rapid cooling (car windows).

  • Laminated Glass: Has plastic layers to prevent shattering (windshields).

Comparing Glass Fragments

Glass evidence is classified by individual and class characteristics.

• Class Characteristics: General properties shared by multiple glass samples.

  • Example: A bottle and a window may both contain soda-lime glass.

• Individual Characteristics: Unique features that can match a fragment to a source.

  • Example: If two glass fragments fit together like puzzle pieces, they came from the same object.

Glass – Measuring and Comparing Density

Flotation Method

A technique for identifying glass by its density.

• Process: A glass sample is placed in a liquid, and the liquid’s density is adjusted until the glass neither sinks nor floats, matching the glass’s density.

• Example: If a crime scene glass fragment matches a broken window’s density, it may have originated from the same source.

Classification of Glass Samples

Glass is categorized based on composition and manufacturing process:

1. Soda-Lime Glass – Most common, used in bottles and windows.

2. Borosilicate Glass – Heat-resistant (Pyrex cookware, lab glassware).

3. Tempered Glass – Strengthened by heat treatment (car side windows).

4. Laminated Glass – Two layers with plastic in between (windshields).

• Forensic Example: A forensic analyst can determine if a car accident victim went through a windshield based on laminated glass found in their clothing.

Trace Elemental Composition of Glass

Using spectroscopy and chemical analysis, forensic scientists identify unique trace elements in glass samples.

• Methods:

  • X-ray Fluorescence (XRF): Detects elements in glass.

  • Inductively Coupled Plasma Mass Spectrometry (ICP-MS): Identifies tiny amounts of trace elements.

• Example: If a burglary suspect’s clothing contains trace glass with the same chemical profile as a crime scene window, it strengthens the case against them.

Radial Fractures: Straight cracks that radiate outward from the point of impact, resembling the spokes of a wheel.

Concentric Fractures: Circular cracks around the impact point, connecting radial fractures.