Determination of Refractive Index and Dispersion of Selected Nigerian Vegetable Oils

Vegetable oils produced in Nigeria from palm, groundnut, coconut, and soybean are important agricultural commodities with potential photonic and optical applications in addition to their nutritional and industrial uses. The refractive index and dispersion characteristics of oils are fundamental optical properties that determine their interaction with light across the visible spectrum. This study determines the refractive index and spectral dispersion of six Nigerian vegetable oils sourced from manufacturers in Ogun State using an Abbe refractometer at five standard wavelengths corresponding to the Fraunhofer spectral lines. The Cauchy dispersion equation is fitted to the wavelength-dependent refractive index data for each oil. Abbe number, which characterises the optical dispersion, is computed. Results indicate refractive indices ranging from 1.456 for refined coconut oil to 1.472 for crude palm oil at the sodium D-line wavelength of 589 nm. Abbe numbers range from 38 to 47, classifying all tested oils as dense flint optical media by the Abbe diagram. The higher refractive index of crude oils relative to their refined counterparts reflects the contribution of free fatty acids, tocopherols, and carotenoid pigments. These data are useful for designing optical instruments and sensors employing natural liquid media in low-cost laboratory settings. Keywords: refractive index, dispersion, vegetable oil, Abbe refractometer, optical properties