What are optically denser and optically rarer media?

(N/A) An optically denser medium is one in which the speed of light is lower,and it has a higher refractive index. Conversely,an optically rarer medium is one in which the speed of light is higher,and it has a lower refractive index.
When light travels from an optically rarer medium (medium-$1$) to an optically denser medium (medium-$2$),the refractive index of medium-$2$ with respect to medium-$1$ is $n_{21} > 1$.
According to Snell's law:
$\frac{\sin i}{\sin r} = n_{21}$
Since $n_{21} > 1$,we have:
$\frac{\sin i}{\sin r} > 1$
$\therefore \sin i > \sin r$
$\therefore i > r$
This implies that the refracted ray bends towards the normal when entering a denser medium from a rarer medium.