Let speed of light in denser medium is $v_{1}$ and in rarer medium is $v_{2}$ and $v_{2}>v_{1}$.

Refracted ray moves away from the perpendicular when it goes from the denser medium to rarer medium.

At certain incident angle, angle of refraction becomes $90^{\circ}$. This angle is called critical angle $i_{c}$

From Snell's law,

$n_{1} \sin i=n_{2} \sin r$

If $i=i_{c^{\prime}}$, then $r=90^{\circ}$ hence $\sin 90^{\circ}=1$

$\therefore \sin i_{c}=\frac{n_{2}}{n_{1}}$

We will not find any refraction ray for all the angles greater than the critical angle.

Rarer medium for which $v_{2}>v_{1}$, the refraction of incident plane on it refracted away from perpendicular of plane wave. This is shown in figure.