The index of refraction of vacuum is 1, since light travels at the speed of light \(c\) so \(v = c\) and \(n = c/c = 1\). In any material medium, light travels slower than \(c\), so \(v < c\) and \(n > 1\). The greater the value of the index of refraction \(n\) in a given medium, the slower the speed \(v\) at which light propagates in that medium. Table 23-1 lists the index of refraction of some common materials. Note that the index of refraction of air is equal to one to three significant digits, so we’ll often take \(n_{\mathrm{air}} = 1\) in calculations.