Light is incident normally on the face $\mathrm{AB}$ of a prism as shown in the figure. A layer of liquid of refractive index $\mu$ is applied on the face $\mathrm{AC}$ of the prism. If the prism is made of glass of refractive index $\frac{3}{2}$, then the limit of $\mu$ for which total internal reflection takes place on face $\mathrm{AC}$ is