# Define relative refractive index

The ratio of the speed of light in a vacuum to the speed of monochromatic light in the substance of interest. Also called the absolute index of refraction it is commonly called the refractive index or index of refraction. However, those two terms are also sometimes applied without warning to the ration found when the ray emerges into the air instead of a vacuum. That ratio should be called a relative refractive index.

The ratio of the first value to the second one is about the relative refractive index of the glass. In the case of a concave surface, beams originally propagating along the main optical axis will be refracted in such a way that they will never intersect.