If the critical angle for total internal reflection from a medium to vacuum is $30^o$. Then velocity of light in the medium is:

Critical angle for total internal reflection ${\theta }_c=30^o$

As we know, refractive index of air medium $n_{air}=1$

Refractive index of the medium $n=\frac{1}{\sin {\theta }_c}$

$\therefore$ $n=\frac{1}{\sin 30^o}=\frac{1}{0.5}$

$⟹$ $n=2$

Velocity of light in medium $v=\frac{c}{n}$

$\therefore$ $v=\frac{3\times 10^8}{2}=1.5\times 10^{8}m/s$