Malus' law states that when **linearly polarised** light of intensity I₀ passes through a polaroid whose pass-axis makes angle θ with the incident polarisation, the transmitted intensity is:
AI = I₀ sin²θ
BI = I₀ cos²θ — zero when θ = 90° (crossed polaroids)
CI = I₀ tan²θ
DI = I₀ (1 + cos θ)/2
Answer & Solution
Correct answer: B. I = I₀ cos²θ — zero when θ = 90° (crossed polaroids)
**Malus' law: I = I₀ cos²θ**. At θ = 0: full transmission. At θ = 90° (crossed polaroids): zero transmission. At θ = 45°: half. The cos²θ dependence comes from the projection of E_polarised onto the pass-axis: only E cos θ passes, and intensity ∝ E².
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