The refractive index of a substance is the ratio of the speed of light in a vacuum to the speed of light in the substance. For practical measurements, including this method, the scales of standard instruments indicate refractive indices with respect to air, rather than vacuum. The index of refraction of oils is characteristic within certain limits for each kind of oil. It is related to the degree of unsaturation, but it is affected by other factors, such as free fatty acid content, oxidation, and heat treatment. Applicable to all normal oils and liquid fats.