Refractive Index (RI)

Introduction

Refractive index (RI) is a unitless measure that indicates the extent to which a light beam changes direction while traveling through a substance. It is calculated by dividing the speed of light in a vacuum by the speed of light in another medium (solid, liquid, or gas). Accurately using the appropriate refractive index for a suspension and particle is essential for obtaining accurate particle size distributions through light scattering techniques such as laser diffraction.
Laser diffraction in particle size distribution analysis often involves the application of either the Mie theory or the Fraunhofer approximation to interpret the scattering matrix and determine particle sizes on a volume basis. When applying the Mie theory, it is essential to understand the optical characteristics of the dispersant absolute refractive index only) and the material being tested (actual refractive index and imaginary component). It is frequently possible to establish the absolute refractive indices of common materials from published data.
The International Organisation for Standardisation (ISO)13320:2009 (Particle Size Analysis – Laser Diffraction) defines a particle’s complex refractive index as having both a real and an imaginary (absorption) component. A Becke line test may measure particles whose accurate refractive index is unknown. The actual refractive index of uncommon dispersants for which literature values are unknown can be determined using a refractometer.
A particle’s fictional absorption component represents the reduction in a light beam’s intensity that is not caused by scattering. The absorption value can be affected by particle characteristics such as surface texture, roughness, and reflectivity; internal reflection or refraction; or color and opacity (i.e., absolute absorption). One can calculate a suitable imaginary value by evaluating the laser diffraction analysis assessment of the fit between the modeled and actual data.

Advantages

• Ability to analyze liquids, crystals, and solids
• There are numerous techniques, such as the Becke Line, Dissolved Solids, and Refractometer.
• Programmable refractometer for temperature

Considerations

• The preferred test material is crystalline
• Solubility profile helpful for Dissolved Solids technique
• Only tests at a wavelength of 589.3 nm were completed.

Video 01: 5-05 QC RI Refractive Index