What measures how greatly a substance reduces the speed of light?

The refractive index of a substance quantifies how much the speed of light is reduced as it travels through that material compared to its speed in a vacuum. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the substance. This measure is crucial in optics because it affects how light rays bend when entering different media, which is fundamental for understanding phenomena such as refraction and the design of lenses.

Optical density, while related to the absorption of light, does not directly convey the speed reduction in the same manner. It is more focused on the extent to which a material can transmit light without scattering, rather than how fast light travels through it. The light speed ratio and velocity factor are terms that are less commonly used in this context and do not provide a standard method for defining the reduction of light speed in materials. Thus, the refractive index remains the standard and most relevant measure for this property in optics.