Answer: Same frequency and constant phase difference.
- A Same frequency and constant phase difference
- B Identical amplitude mainly, with phase free to drift randomly
- C Identical wavelength mainly, with frequency free to vary
- D Identical intensity mainly, with little constraint on phase
Correct answer: A. Same frequency and constant phase difference
Explanation: Coherent sources must have the same frequency and maintain a constant phase difference. Only coherent sources can produce stable interference patterns.
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.