In passing through a boundary, 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 $i>{\sin }^{-1}\left(\frac{{\mu }_R}{{\mu }_D}\right)$ or $i>i_c$(critical angle), then also there will be no refraction of light and light will be totally internally reflected.