Answer: Both slits have equal intensities.
- A Both slits have equal intensities
- B One slit is blocked
- C The slits are very far apart
- D The screen is very close
Correct answer: A. Both slits have equal intensities
Explanation: Visibility = (I<sub>max</sub> - I<sub>min</sub>)/(I<sub>max</sub> + I<sub>min</sub>). Maximum visibility = 1 occurs when both sources have equal intensity (I<sub>1</sub> = I<sub>2</sub>), giving I<sub>min</sub> = 0.
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.