For a hollow prism filled with a liquid, the refractive index \( \mu \) of the liquid is related to the minimum deviation \( \delta \) and the angle of the prism \( A \) by the formula:
\[
\mu = \sin\left( \frac{A + \delta}{2} \right)
\]
Given that the minimum deviation \( \delta = 30^\circ \), we have:
\[
\mu = \sin\left( \frac{30^\circ + 30^\circ}{2} \right) = \sin(30^\circ) = \frac{1}{2}
\]
Thus, the refractive index \( \mu \) is \( \sqrt{2} \).