Answer: Dark (due to half-wave loss on reflection).
- A Dark (due to half-wave loss on reflection)
- B Bright, exactly as in the ordinary double-slit setup
- C Not present, unlike the ordinary double-slit setup
- D Distinctly coloured even under generally monochromatic light
Correct answer: A. Dark (due to half-wave loss on reflection)
Explanation: In Lloyd's mirror, one beam reflects from the mirror with a phase change of π (half-wave loss). At zero path difference, the two beams are out of phase, so the central fringe is dark.
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.