refractive index - ratio of the velocity of light at a specified wave length in air to its velocity in a substance under examination. The refractive index of light-colored petroleum liquids can be determined by test method ASTM D 1218, using a refractometer and a monochromatic light source. Refractive index is an excellent test for uniform composition of solvents, rubber process oils, and other petroleum products. It may also be used in combination with other simple tests to estimate the distribution of naphrhenic, paraffinic, and aromatic carbon atoms in a process oil.