The refractive index, also known as the index of refraction, is a fundamental property of a material that describes how light propagates through it. It is a dimensionless quantity that measures the speed of light in a vacuum compared to its speed in the material. Mathematically, the refractive index (n) of a material is defined as the ratio of the velocity of light in a vacuum (c) to its velocity in the material (v):

n = c/v