Answer: Plane wavefront.
- A Plane wavefront
- B Spherical wavefront
- C Cylindrical wavefront
- D Elliptical wavefront
Correct answer: A. Plane wavefront
Explanation: At large distances from a point source, the spherical wavefront has such a large radius that a small portion appears flat (plane wavefront). This is why parallel rays are assumed from distant sources.
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.