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If n₂₁ is the refractive index of medium 2 w.r.t. medium 1, then n₁₂ equals:
An₂₁
B1 / n₂₁
Cn₂₁²
D1 − n₂₁
Answer & Solution
Correct answer: B. 1 / n₂₁
**n₁₂ = 1 / n₂₁** (reversibility of light). Also, for three media: **n₃₁ = n₃₂ × n₂₁** (or equivalently n₃₂ = n₃₁ × n₁₂).
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