Answer: keeps changing randomly.
- A remains constant in time
- B keeps changing randomly
- C is exactly zero
- D is exactly 90°
Correct answer: B. keeps changing randomly
Explanation: Coherence needs a constant phase relationship. Independent sources emit light in random bursts, so their phase difference varies randomly and no steady interference is seen.
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.