How is the refractive index of a material related to the speed of light in that material?

In the field of optics, the refractive index ($n$) of a material is a measure that describes how the speed of light changes as it passes through that medium compared to its speed in a vacuum.1

The relationship is defined by the following equation:

$$n=\frac{c}{v}$$

In this formula:
* $n$ is the refractive index of the material.1
* $c$ represents the speed of light in a vacuum, which is a fundamental constant of approximately 299,792,458 metres per second.123
* $v$ is the velocity (speed) of light of a given wavelength within the specific substance.1

Because the speed of light in a vacuum ($c$) is the maximum possible velocity for light, the refractive index of any material is typically greater than 1, indicating that light travels more slowly in that medium than it does in a vacuum.12