Forensic Medicine · Trace Evidence and Crime Scene Reconstruction (Hair, Fibre, Glass, Paint, GSR)

In forensic glass analysis, the most discriminating physical property used to compare glass fragments from a crime scene and suspect is:

  • A Colour and thickness of the glass
  • B The Becke line direction seen under polarised light
  • C Refractive index (RI), measured by the phase-contrast immersion method ✓
  • D Hardness measured by Mohs scale
Correct answer: C. Refractive index (RI), measured by the phase-contrast immersion method

Explanation

Refractive index (RI) is the most discriminating single physical property for forensic glass comparison. RI is measured using the temperature-variation immersion method (GRIM — Glass Refractive Index Measurement instrument), which detects RI differences as small as 0.0001. Glass with matching RI, elemental composition (by LA-ICP-MS), and fracture pattern provides strong associative evidence. Colour and thickness are too variable and subjective. The Becke line is a phenomenon seen during RI matching but is the observation tool, not the property itself. Mohs hardness is not discriminating enough.

Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.

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