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(B) Initially, the two polarisers $P_1$ and $P_2$ are crossed, meaning the angle between their pass-axes is $90^{\circ}$, which blocks all light.When

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$\frac{3}{32} I_0$

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(B) Initially, the two polarisers $P_1$ and $P_2$ are crossed, meaning the angle between their pass-axes is $90^{\circ}$, which blocks all light.When an unpolarised beam of intensity $I_0$ passes through the first polariser $P_1$, the intensity of the emerging light is $I_1 = \frac{I_0}{2}$.$A$ third polariser $P_3$ is introduced between $P_1$ and $P_2$ at an angle $	heta_1 = 60^{\circ}$ with respect to $P_1$.Using Malus's Law, the intensity $I_3$ emerging from $P_3$ is $I_3 = I_1 \cos^2(60^{\circ}) = \frac{I_0}{2} 	imes (\frac{1}{2})^2 = \frac{I_0}{8}$.The angle between the pass-axes of $P_3$ and $P_2$ is $	heta_2 = 90^{\circ} - 60^{\circ} = 30^{\circ}$.The final intensity $I_f$ emerging from $P_2$ is $I_f = I_3 \cos^2(30^{\circ}) = \frac{I_0}{8} 	imes (\frac{\sqrt{3}}{2})^2 = \frac{I_0}{8} 	imes \frac{3}{4} = \frac{3}{32} I_0$.

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