Answer: 200 nm.
- A 200 nm
- B 100 nm
- C 300 nm
- D 400 nm
Correct answer: A. 200 nm
Explanation: Dark in reflection (destructive for reflected): 2nt = mλ. Minimum (m=1): t = λ/2n = 600/(2×1.5) = 200 nm.
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.