Regardless of its frequency, any electromagnetic radiation (light) travels at \(c=3\times 10^8\; m/s\) in a vacuum. However, the speed of light decreases to a value \(c_B\) when propagating through a homogeneous material medium. The spare part index \(n_B\) is defined as the quotient between both speeds:

The following table shows refractive index values for different substances. The sodium D line (589.3 nm) at 25°C and 1 atm is used as radiation.