Refractive Index (RI)
Definition
A physical property of glass measuring how much light bends as it passes through. RI is the primary discriminating characteristic used in forensic glass comparisons. Population frequency data for RI values exist in national databases but differ in size and composition between jurisdictions.
- Range in common glasses
- 1.47 (borosilicate) to 1.58 (lead crystal)
- Measurement standard
- Measured at sodium D-line (589 nm wavelength)
- Definition
- Ratio of light speed in vacuum to light speed in the material
Common questions
What exactly is refractive index and why does it matter in forensics?+
Refractive index is a number that measures how much a material bends light passing through it. It is calculated as the speed of light in a vacuum divided by the speed of light in the material. In forensics, especially for glass evidence, RI is one of the most important properties because glasses from different sources have different RIs that help link or exclude samples.
How much does refractive index vary between different types of glass?+
Common glasses range from about 1.47 for borosilicate glass up to 1.58 for lead crystal. Even small differences in RI between two glass samples can suggest they came from different sources, which is why forensic examiners measure RI carefully using techniques like the Becke line test or hot-stage microscopy.
What is the Becke line and how does it relate to refractive index?+
The Becke line is a bright halo that appears at the edge of a glass particle when viewed under a microscope. It forms because of how light bends at the interface between glass and the surrounding medium, and the position and behavior of this line help examiners determine whether the glass and medium have similar RIs. This technique can be applied using a standard microscope rather than the more specialized equipment required for hot-stage methods.
Related terms
- ENFSI ENG3-2013
- The ENFSI Glass Expert Working Group guideline for glass comparison casework; specifies the examination sequence, within-source variation requirements, statistical comparison framework, and...
- ASTM E1967
- The ASTM International standard for forensic glass RI measurement; specifies three acceptable methods (automated, semi-automated, manual), minimum precision of ±0.0001 RI units,...
- ASTM E2927
- Standard test method for the determination of trace elements in glass samples using LA-ICP-MS; specifies the element suite, calibration standards (NIST SRM...
- Bayesian Likelihood Ratio
- A statistical framework for expressing the evidential value of a comparison: the ratio of the probability of the observed RI similarity given...
- Becke Line
- The bright optical halo at the edge of a solid fragment immersed in a liquid, visible under the microscope; moves toward the...
- Cauchy Equation
- The empirical equation n(λ) = A + B/λ² + C/λ⁴ that describes the wavelength dependence (dispersion) of a material's refractive index; used...
- Dispersion
- The variation of refractive index with wavelength; all glasses and immersion oils show dispersion, which is why RI must be specified at...
- Float Process
- The Pilkington 1959 manufacturing method in which molten glass is poured onto a bath of molten tin, producing a ribbon with two...
- Foundational Validity
- The requirement, articulated in the 2009 US National Academy of Sciences report and taken up by courts and reform bodies, that a...
- Glass-Transition Temperature (Tg)
- The temperature below which a glass behaves as a rigid solid rather than a viscous liquid; a fundamental thermal property that varies...
- GRIM-3
- Glass Refractive Index Measurement, third-generation instrument by Foster + Freeman (UK); an automated phase-contrast hot-stage system that identifies the RI match point...
- Immersion Method
- The forensic technique of matching a glass fragment's RI to that of a reference oil by adjusting temperature; the fragment becomes invisible...
Explained in these topics
- Descriptive Statistics for Forensic DataThe ratio of the speed of light in a vacuum to its speed in a material. For glass evidence, RI is the primary continuous measurement used to compare fragments;...
- Glass: Types, Properties and ManufactureThe ratio of the speed of light in vacuum to its speed in the glass medium; at the sodium D-line (589 nm) this is the primary physical comparison parameter for...
- Match Probabilities Beyond DNAA physical property of glass measuring how much light bends as it passes through. RI is the primary discriminating characteristic used in forensic glass compar...
- Refractive Index: Becke Line, GRIM and Mettler Hot StageThe ratio of the speed of light in vacuum to its speed in the medium (n = c/v); a dimensionless number that characterises the optical density of a material and...