Answer: 2nt = (2m+1)λ/2 (half-wave loss at first surface).
- A 2nt = (2m+1)λ/2 (half-wave loss at first surface)
- B 2nt = mλ, the condition ignoring the half-wave phase loss
- C 2t = mλ/n, omitting the refractive index from the optical path
- D 2nt = mλ/2, half the correct optical path condition
Correct answer: A. 2nt = (2m+1)λ/2 (half-wave loss at first surface)
Explanation: At the top surface (denser medium), reflected ray has half-wave loss (π phase shift). At the bottom, no phase shift. For constructive interference: 2nt = (m + 1/2)λ, i.e., odd multiples of λ/2.
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.