Refractive index (n) is a dimensionless number that describes how light propagates through a medium. It is defined as the ratio of the speed of light in vacuum to the speed of light in the medium. In the context of a Safety Data Sheet (SDS), refractive index serves as a characteristic physical property that can be used for substance identification, purity assessment, and quality control.
The refractive index is defined by the equation:
Where:
When light passes from one medium to another, it changes direction according to Snell's law:
Where:
Key concepts related to refractive index include:
Refractive index information in an SDS is important for several reasons:
Several techniques are used to measure refractive index:
| Method | Description | Typical Applications |
|---|---|---|
| Refractometer | Measures the critical angle of total internal reflection | Liquids, solutions, transparent solids |
| Abbe Refractometer | Uses critical angle principle with compensation for dispersion | Precise measurements for liquids and solutions |
| Digital Refractometer | Automated measurement using critical angle principle | Routine quality control, field measurements |
| Immersion Method | Observes visibility of a solid immersed in liquids of known refractive index | Transparent solids, powders, fibers |
| Interferometry | Measures phase shifts in light waves | Thin films, optical materials, high precision measurements |
| Ellipsometry | Measures changes in polarization state of light reflected from a surface | Thin films, surfaces, complex refractive index |
| Becke Line Method | Microscopic technique observing bright line at particle boundaries | Small particles, minerals, forensic analysis |
| ASTM Methods | Standardized procedures (e.g., ASTM D1218, D1747) | Regulatory testing, specification compliance |
Refractive index values are typically reported with specific notation:
| Notation | Description | Example |
|---|---|---|
| nD | Refractive index measured at the sodium D line (589.3 nm) | nD = 1.3330 (water at 20°C) |
| nD20 | Refractive index measured at the sodium D line and 20°C | nD20 = 1.3330 (water) |
| nF, nC | Refractive indices at the hydrogen F line (486.1 nm) and C line (656.3 nm) | Used for calculating dispersion |
| n(λ) | Refractive index as a function of wavelength λ | Dispersion relation |
| n + ik | Complex refractive index, where k is the extinction coefficient | For absorbing materials |
| ne, no | Extraordinary and ordinary refractive indices for birefringent materials | For anisotropic crystals |
The most commonly reported value in SDSs is nD20, the refractive index measured at the sodium D line (589.3 nm) and 20°C.
| Substance | Refractive Index (nD20) | Category | Notes |
|---|---|---|---|
| Air | 1.000293 | Very Low | At standard conditions |
| Water | 1.333 | Low | Reference liquid |
| Ethanol | 1.361 | Low | Common alcohol |
| Glycerol | 1.473 | Moderate | Viscous alcohol |
| Olive Oil | 1.47 | Moderate | Typical vegetable oil |
| Benzene | 1.501 | Moderate | Aromatic hydrocarbon |
| Acrylic (PMMA) | 1.49 | Moderate | Common plastic |
| Crown Glass | 1.52 | High | Common optical glass |
| Polycarbonate | 1.586 | High | Used in eyewear |
| Carbon Disulfide | 1.628 | High | High dispersion liquid |
| Flint Glass | 1.62-1.96 | High | Lead-containing glass |
| Sapphire | 1.762-1.778 | Very High | Anisotropic crystal |
| Diamond | 2.417 | Very High | Highest natural refractive index |
Refractive index generally decreases with increasing temperature due to decreasing density. The temperature dependence can often be approximated by:
Where:
For most liquids, dn/dT is negative and typically in the range of -1×10⁻⁴ to -5×10⁻⁴ per °C.
Refractive index varies with wavelength, a phenomenon known as dispersion. For transparent materials in the visible range, refractive index typically decreases with increasing wavelength. This relationship can often be described by the Cauchy equation:
Where:
For solutions, refractive index typically varies with concentration, often in a nearly linear relationship for dilute solutions:
Where:
Refractive index generally increases with pressure due to increasing density. For most materials, the pressure dependence is relatively small at moderate pressures but can become significant at very high pressures.
The molecular structure significantly affects refractive index:
While refractive index itself is not directly related to chemical hazards, it can have several safety implications:
It's important to note that refractive index measurements typically require direct contact with the sample, which should be performed with appropriate safety precautions based on the hazards of the material being tested.
Refractive index is not specifically mandated by GHS for all substances, but it is often included in Section 9 of Safety Data Sheets as supplementary information that helps with substance identification and quality control.
Refractive index information is particularly relevant for:
While not directly related to hazard classification, accurate substance identification is fundamental to proper hazard communication, making refractive index an important supporting parameter in many cases.
When reporting refractive index in an SDS: