Answer: 5.
- A 3
- B 4
- C 5
- D 2
Correct answer: C. 5
Explanation: d sinθ is the same for both: 4 × 500 = n × 625. 2000 = 625n → n = 3.2... Wait: n = 2000/625 = 3.2. So n=3 gives 3×625=1875 ≠ 2000. Actually n=4 would give 2500. Let me recalculate: 4×500=2000 and 2000/625 = 3.2, not integer. The correct: 4×500 = n×625 → n = 3.2. So n=5 gives 5×400=2000. With λ = 625: n × 625 = 4 × 500 means n = 3.2 (not integer). The option C (n=5) works if: 5 × 400 = 2000, not 625. Correct answer is n = 3 by closest approximation, but precise answer is n not integer. This problem assumes λ_2=500 and λ_1=625: n×625 = 4×500 → n=3.2. For n=4: 4×625=2500=5×500 → n=5 for λ=500. Answer: 5 is correct (5th order of 500nm = 4th order of 625nm means n5×500=n4×625 → 2500=2500).
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.