= Relative refractive index
{title2=$n_{21}=n_2/n_1$}
The <relative refractive index> of medium two with respect to medium one is $n_{21}=n_2/n_1$, using their absolute <refractive indices> at the same <frequency>. For transparent media with <isotropy> and positive <refractive indices>, <Snell's law> gives $n_{21}=\sin i/\sin r$, with the <angles> measured from the interface normal. Thus a <refraction> measurement in air directly determines an index relative to air. Converting it to an absolute <refractive index> requires the <refractive index> of that air; replacing it by one is an approximation.
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