GKFix MCQ Question

If n₂₁ is the refractive index of medium 1 with respect to medium 2, what is its relation to n₁₂?

• A. n₂₁ = n₁₂
• B. n₂₁ = 2n₁₂
• C. n₂₁ = 1/n₁₂
• D. n₂₁ = n₁₂²