Light emerges out of a convex lens when a source of light is kept at its focus. The shape of the wavefront of the emerging light is-
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Plane
A wavefront is always perpendicular to the rays. Light from a point source at the focus of a convex lens emerges as a beam of rays parallel to the principal axis. Surfaces perpendicular to parallel rays are planes, so the emerging wavefront is plane.
In wave terms, the lens converts the diverging spherical wavefront from the focus into a plane wavefront.
Why the other options are incorrect
Option (1) spherical diverging — Incorrect. A spherical diverging wavefront is what leaves the point source itself, before it passes through the lens.
Option (2) Cylindrical — Incorrect. A cylindrical wavefront comes from a linear source such as a slit.
Option (3) Spherical converging — Incorrect. A spherical converging wavefront emerges when a plane wave (a parallel beam) falls on the convex lens, the reverse of this situation.
NCERT (2026-27):
Exercises – …
