Answer: 222.
- A 222
- B 111
- C 500
- D 333
Correct answer: A. 222
Explanation: The region between the two direct beam spots has angular extent θ ≈ d/D = 0.2mm/1500mm ≈ 1/7500 rad from each slit's direction. Fringe spacing = λD/d = 600nm × 1.5m / 0.2mm = 4.5mm. Width of central region ≈ D (point of second slit on screen) = 0.2 mm... Actually the question refers to the central diffraction envelope. Bright fringes between minima: N = d/a for single slit envelope. Assuming slit width a = d/500: N ≈ 500.
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.