Relative angle between polarizers determines transmission

Steps:

• Initial filter axes parallel (0° relative angle), allowing maximum transmission if aligned with light.
• One filter rotates clockwise by θ, other anticlockwise by θ, creating 2θ relative angle between axes.
• By Malus's law, transmitted intensity ∝ cos²(relative angle) = cos²(2θ).
• At θ=90°, 2θ=180°, cos²(180°)=1, full transmission between filters, brightest.
• At θ=180°, 2θ=360°=0° equivalent, cos²(0°)=1, full transmission between filters, brightest.

Why C is correct:

• Relative angles of 180° and 360° are equivalent to 0° or 180° (parallel axes), yielding cos²=1 per Malus's law for maximum intensity.

Why the others are wrong:

• A: 180° rotation yields brightest, not darkest (cos²(360°)=1).
• B: 90° rotation yields brightest, not darkest (cos²(180°)=1).
• D: Both cases brightest, not darkest.

Final answer: C