Comparatively Speaking: Refractive Index vs. Refraction of Light

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.

According to Wikipedia, the following general rules apply to how light interacts with different materials:
Light arriving at an opaque surface will be reflected, scattered or absorbed in some combination.
• The color of opaque objects that do not reflect is determined by the wavelengths of light that are scattered. If objects scatter all wavelengths, they appear white. If they absorb all wavelengths, they appear black.
• Opaque objects that reflect light of different wavelengths with different efficiencies look like tinted mirrors with their colors determined by the differences in efficiency.
• Objects that transmit light are translucent (scattering the transmitted light) or transparant (not scattering the transmitted light). If they also absorb (or reflect) light of varying wavelengths differentially, they appear tinted with a color determined by the nature of that absorption (or that reflectance).
• Objects may emit light that they generate themselves, rather than merely reflecting or transmitting light. They may do so due to their elevated temperature, which are then said to be incandescent; as a result of certain chemical reactions, a phenomenon called chemoluminescence; or for other reasons.
• Objects may absorb light and then as a consequence emit light that has different properties. They are then called fluorescent (if light is emitted only while light is absorbed) or phosphorescent (if light is emitted even after light ceases to be absorbed; this term is also sometimes loosely applied to light emitted due to chemical reactions).

A popular example of dispersion of light is a rainbow. A rainbow results when light enters water droplets. It is refracted, totally internally reflected off the back, and refracted again as it leaves. The refractions cause the dispersion effect and a rainbow results. When light refracts, different colors refract through different angles. This dispersion effect produces a rainbow of color.2


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