Answer: nλ_1 = (n+1)λ_2.
- A nλ_1 = (n+1)λ_2
- B λ_1 = λ_2
- C nλ_1 = nλ_2
- D (n+1)λ_1 = nλ_2
Correct answer: A. nλ_1 = (n+1)λ_2
Explanation: For overlap: d sinθ must be equal. nλ_1 = (n+1)λ_2. This means λ_2 = nλ_1/(n+1). This is why free spectral range matters in spectroscopy.
Two coherent slits S₁ and S₂ act as secondary sources; at each point on the screen, the path difference between light from S₁ and S₂ determines whether the waves arrive in phase (bright fringe) or out of phase (dark fringe), producing the characteristic alternating fringe pattern.
Concept context
Huygens principle, Young's double slit, diffraction, polarization. Essential for JEE.