Given below are two statements: one is labelled as Assertion (A)...

Assertion (A) is true: As the analyser rotates through $360^{\circ}$, the angle $\theta$ between the transmission axes of the polariser and the analyser changes from $0^{\circ}$ to $360^{\circ}$. The transmitted intensity is zero when the axes are perpendicular (crossed polarisers), which occurs at $\theta = 90^{\circ}$ and $\theta = 270^{\circ}$. Thus, the intensity becomes zero exactly twice in one complete rotation.

Reason (R) is true: By Malus's law, the transmitted intensity is $I = I_0 \cos^2\theta$. The mathematical zeroes of $\cos\theta$ in the interval $[0^{\circ}, 360^{\circ})$ are at $90^{\circ}$ and $270^{\circ}$, which directly explains why the intensity becomes zero twice.

Therefore, both (A) and (R) are true and (R) is the correct explanation of (A).

Answer: Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A)