(a) State the two conditions under which total internal reflection occurs. (b) A transparent container contains layers of three immiscible transparent liquids A, B and C of refractive indices $n$, $\\frac{3}{4}n$ and $\\frac{2}{3}n$, respectively. A laser beam is incident at the interface between A and B at an angle $\ heta$ as shown in figure. Prove that the beam does not enter region C at all for $\sin \ heta \ge \\frac{2}{3}$.