Answer: 100 nm.
- A 200 nm
- B 100 nm
- C 133 nm
- D 50 nm
Correct answer: B. 100 nm
Explanation: For brightness in reflection 2nt = λ/2 (first order), so t = λ/(4n) = 532/(4 × 1.33) = 100 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.