Comparatively Speaking: Refractive Index vs. Refraction of Light

By: Anthony O'Lenick, Jr., Siltech LLC

Posted: December 20, 2010

Tony O'Lenick explains the difference between the refractive index and refraction of light to produce color. These terms can be used by the formulator to produce transparent emulsions and predict the color of emulsions, respectively.

Refractive Index

The refractive index, also called index of refraction, relates to the change in the speed of light passing through different media. Refractive index expresses a ratio of the speed of light in vacuum, relative to that in the considered medium. The velocity of light in a vacuum is a constant, and it travels slower through other transparent media.

Formulators should familiarize themselves with the concept of refraction, as transparent emulsions can be made by matching the refractive index of the oil and water components. This can be done with additives to the water phase, like glycerin.

One project that illustrates the concept of refractive index is lowering a glass rod into an oil, where it seems to vanish. This is the result of refractive index matching. Another example of refractive index is shown in Figure 1, where the rod appears to bend because air and the solvent (water) have different refractive indices. Table 1 shows the refractive indices of several commonly used cosmetic ingredients.

Refraction of Light–Color

The way in which light interacts with substrates results in an important cosmetic effect—color. Color is an important factor in how personal care products are perceived by consumers. It is fundamentally the first attribute a consumer sees when applying a product; odor is generally the second. Most people know that passing white light through a prism will divide it into its component colors and the light will display a rainbow of red through violet. The way in which light interacts with different materials is key to the color produced by that material.