Three polarizers, centered on a common axis and with their planes parallel to one another, have transmission axes oriented at angles of $\theta_{1}, \theta_{2}$, and $\theta_{3}$ from the vertical, as shown in Figure $\mathrm{P} 24.59$. Light of intensity $I_{i}$, polarized with its plane of polarization oriented vertically, is incident from the left onto the first polarizer. What is the ratio $I_{f} / I_{i}$ of the final transmitted intensity to the incident intensity if (a) $\theta_{1}=45^{\circ}, \theta_{2}=90^{\circ}$, and $\theta_{3}=0^{\circ}$ ? (b) $\theta_{1}=0^{\circ}$,
$\theta_{2}=45^{\circ}$, and $\theta_{3}=90^{\circ}$ ?