Answer: Photoelectric effect.
- A Photoelectric effect
- B Interference in most textbook accounts
- C Diffraction during normal conditions
- D Polarization as generally observed
Correct answer: A. Photoelectric effect
Explanation: The photoelectric effect requires the particle (photon) nature of light. Wave theory predicts no intensity threshold, but experimentally there is a definite threshold frequency.
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.