a) $I=\left(\frac{P_{0}}{4 \pi d^{2}}\right) \cos ^{2} \theta=\left(\frac{24 \mathrm{~W}}{4 \pi(2 \mathrm{~m})^{2}}\right)\left(\frac{1}{2}\right)^{2}=0,125 \mathrm{~W} / \mathrm{m}^{2}$ b) $n_{1} \operatorname{sen} \theta_{1}=n_{2} \operatorname{sen} \theta_{2}$ $n_{2}=\frac{n_{1} \operatorname{sen} \theta_{B}}{\operatorname{sen} \theta_{r}}=\frac{n_{1} \operatorname{sen} \theta_{B}}{\operatorname{sen}\left(90^{\circ}-\theta_{B}\right)}=\frac{1 \times \operatorname{sen} 60^{\circ}}{\operatorname{sen} 30^{\circ}}=\frac{\sqrt{3} / 2}{1 / 2}=\sqrt{3}$