The Index of [[Refraction]], denoted $\eta$ of some substance is a [[Scalar|scalar]] dimensionaless quantity of the [[Wave Speed|Speed]] of [[Electromagnetic Radiation|Light]] in that substance. $\huge \eta = \frac{c}{v} $ Where $v$ denotes the [[Wave Speed]] of light within that material. The [[Index of Refraction]] of a [[Vacuum]] is $1$, and the index of refraction of air is *approximately* $1$. $\huge \eta_{\text{air}} \approx 1 $