In a Young’s double-slit experiment the source emits two wavelengths λ₁ = 400 nm and λ₂ = 600 nm. The 5th bright band of λ₁ coincides with the nth bright band of λ₂. The value of n is
Correct: A
Condition for overlap: m₁λ₁ = m₂λ₂ → 5·400 = n·600 → n = 2000/600 = 3.33. Since only integer orders count, the first common bright fringe occurs at the 15th order of λ₁ and 10th of λ₂; thus the 5th of λ₁ coincides with the 3rd of λ₂.