Malus's law governs transmitted intensity through a polarizer Steps:

• Initial voltmeter reading 3.5 V corresponds to maximum intensity I₀ when transmission axis aligns with vertical polarization (θ = 0°).
• After rotation by θ, transmitted intensity I = I₀ cos²θ, with reading 2.2 V proportional to I.
• Ratio I/I₀ = 2.2/3.5 ≈ 0.629, so cos²θ = 0.629.
• Thus, cosθ = √0.629 ≈ 0.793, θ = arccos(0.793) ≈ 39°. Why B is correct:
• Malus's law I = I₀ cos²θ directly yields θ ≈ 39° for the given intensity ratio. Why the others are wrong:
• A: cos²25° ≈ 0.821 gives 0.821 × 3.5 ≈ 2.9 V, higher than 2.2 V.
• C: cos²51° ≈ 0.396 gives 0.396 × 3.5 ≈ 1.4 V, lower than 2.2 V.
• D: cos²52° ≈ 0.379 gives 0.379 × 3.5 ≈ 1.3 V, lower than 2.2 V.

Final answer: B