Answer: 6β for 400 nm.
- A 6β_600 = 4β_600 = bright fringe at 3 mm if D=1m, d=0.2mm
- B 4β for 600 nm
- C 6β for 400 nm
- D 12β for both
Correct answer: C. 6β for 400 nm
Explanation: n<sub>1</sub> λ_1 = n<sub>2</sub> λ_2. 400 n<sub>1</sub> = 600 n<sub>2</sub>. Minimum: n<sub>1</sub> = 3, n<sub>2</sub> = 2. Position = n<sub>1</sub> λ_1 D/d = 3 × 400 nm × D/d.
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.