The refractive index of a medium, denoted as \(n_m\), is defined as the ratio of the speed of light in vacuum (or air), \(c\), to the speed of light in that medium, \(v\). The relationship is expressed as: \(n_m = \frac{\text{Speed of light in air }}{\text{Speed of light in the medium }}= \frac{c}{ v}\).
The refractive index of diamond is 2.42. This value indicates that light travels 2.42 times slower in diamond than it does in air.