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Refractive Index

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p442-443

2026-08-19

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Refractive Index

Spectroscopy, the property of refraction was discussed. Recall that refraction occurs when light passes through a transparent medium or object. The light bends away from its path and slows down. Refraction is much more pronounced in solids than liquids or gases. Glass is an excellent medium for exhibiting refraction. The amount of refraction caused by glass is an important physical property for the comparison of known and unknown exhibits. Refractive index (n) is the ratio of the velocity of light in a vacuum to the velocity as it passes through the medium. The refractive index of air is negligible and can be used in place of a vacuum.

Refractive index is always greater than one because light travels fastest in a vacuum. For glass, the range of refractive indices is usually between 1.4 and 1.7. Different types of glass have different refractive indices, so this property can be valuable in determining what type of glass is present and for comparing glass fragments. There are two other important properties of refractive index. One is its variation with temperature. Refractive index is inversely proportional to temperature.

N ∝ 1/T This means that, as the temperature is raised, the refractive index decreases. Over a range of perhaps 30° this effect is quite pronounced with liquids, but solids such as glass, exhibit almost no variation. The other important property of refractive index is its variation with wavelength of light. It is also inversely proportional to wavelength.

N ∝ 1/λ How is refractive index measured in a forensic science laboratory? Clearly it is not possible to measure the refractive index directly as it would be impractical to try to measure the speed of light as it passes through a piece of glass. Instead an indirect method must be used. There are a number of indirect methods used in forensic science laboratories. The most popular are immersion methods and the most commonly used of these is the Becke line method.