Refractive Index (RI)

Introduction

Simply explained, the refractive index (RI) is a dimensionless quantity that expresses how much a light beam bends when passing through a material. It is determined by dividing the speed of light in a vacuum by the speed of another media (a solid, liquid, or gas). Understanding and applying the correct refractive index of a suspension and particle is crucial if light scattering techniques like laser diffraction are to yield representative particle size distributions.

In order to determine the particle size distribution (on a volume basis) from a captured scattering matrix, laser diffraction often uses either the Mie theory or the Fraunhofer approximation. When applying the Mie theory, it is important to understand the optical characteristics of both the dispersant (real refractive index only) and the material being tested (actual refractive index and imaginary component). It is frequently possible to establish the real 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. For particles whose true refractive index is unknown, a Becke line test may be used to measure them directly. The true refractive index of uncommon dispersants for which literature values are unknown can be determined using a refractometer.

The fictional or absorption component of a particle 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., real 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