A transparent cube of side $d$, made of a material of refractive index $\mu_2$, is immersed in a liquid of refractive index $\mu_1\left(\mu_1<\mu_2\right)$. A ray is incident on the face AB at an angle $\theta$ (shown in the figure). Total internal reflection takes place at point $E$ on the face $B C$.Then $\theta$ must satisfy:
