Answer: y 0 (fringes shift toward the source).
- A y<sub>0</sub> (fringes shift toward the source)
- B y<sub>0</sub>/2, half the actual shift produced by the displaced source
- C 2y<sub>0</sub>, twice the actual shift produced by the displaced source
- D y<sub>0</sub> D/d, an expression with the wrong dependence on slit separation
Correct answer: A. y<sub>0</sub> (fringes shift toward the source)
Explanation: When source shifts by y<sub>0</sub>, the path difference to the slits changes. The central fringe (zero path difference) shifts by an amount equal to the source shift projected at the screen. For symmetric setup: shift = y<sub>0</sub> D / (distance from source to slits) = y<sub>0</sub> when source is at same distance D.
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.