Forensic labs identify glass fragments with the GRIM method (Glass Refractive Index Measurement) using the Becke line: a bright halo that outlines any transparent object whose refractive index differs from its surrounding medium. A silicone hot-stage oil is heated on a calibrated stage; its index falls smoothly with temperature:
n_oil(T) = n0 − β·(T − T0)
n0 = 1.5300 at T0 = 20 °C, β ≈ 4.2×10⁻⁴ / °C
Match condition: n_oil(T) = n_glass → Δn → 0
Becke-line rule: raising the focus (increasing the
working distance) moves the bright line INTO the
higher-refractive-index medium.
The bottom ray-diagram panel shows why: the shard's curved edge refracts light like a weak lens, obeying Snell's law n₁sinθ₁ = n₂sinθ₂ with the real n(oil) and n(glass) values above. Whichever medium has the higher index bends the rays toward itself and concentrates the light there — that converging bundle is the physical origin of the Becke line, and it is what shifts sides when you raise or lower the objective.
- Glass sample — switches the fragment's own index (float, container, tempered auto glass, borosilicate, lead crystal), matching real forensic reference values.
- Oil temperature — sets n(oil) via the dispersion law above; the fragment's visible edge, refraction distortion and Becke halo all shrink toward zero as n(oil) → n(glass).
- Search for match — sweeps the stage temperature automatically, the way an examiner brackets in on the disappearance point.
- Raise / lower objective — flips which side of the boundary the Becke line sits on, the diagnostic step that confirms which medium actually has the higher index.
- Drag the microscope field — pans the stage under the objective, just like nudging a slide by hand.
Matching Δn to within about ±0.0002 lets an examiner say a crime-scene fragment and a suspect's clothing fragment are consistent with a common source — one strand of Locard's Exchange Principle, that every contact leaves a trace.