In passing through a boundary from denser to rarer medium refraction will not take place if-

When light is incident normally on the boundary, then $i=0$, So, according to Snell's law ${\mu }_1\sin i={\mu }_2 \sin r, r$ is also zero, i.e., there is no refraction.

If ${\mu }_1={\mu }_2$, then boundary will not be visible and $r=i$, i.e., there will be no refraction. If  $i>{\sin }^{-1}({\mu }_R/{\mu }_D)$ or $i>i_c$(critical angle), then also there will be no refraction of light and light will be totally internally reflected.