The condition for observing diffraction from a single slit is that the light wavefront incident on the slit should be
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Plane
In single-slit diffraction (the Fraunhofer arrangement), a parallel beam of light falls on the slit. Its wavefront is a plane parallel to the slit, so every point of the slit opening is in phase. Each point then sends out secondary wavelets that start together, and the pattern on the screen depends only on the path differences \( a\sin\theta \) from the different parts of the slit; this gives the minima at \( a\sin\theta = n\lambda \).
NCERT (2026-27):
The Single Slit – “… We are treating different parts of the wavefront at the slit as secondary sources. Because the incoming wavefront is parallel to the plane of the slit, these sources are in phase.”