A plane wavefront is incident on a thin convex lens. The nature of wa…

The correct answer is Option (2) → Spherical and converging

A convex lens is thicker at the centre than at the edges. The part of a plane wavefront passing through the centre travels through more glass, where light is slower, so it is delayed more than the parts passing near the edges.

The emerging wavefront is therefore curved, with its edges ahead of its centre. It is a spherical wavefront converging to the focus of the lens.

Option (1) Plane wavefront — Incorrect. A plane wavefront stays plane only through a parallel-sided slab; a lens delays the centre more than the edges.

Option (2) Spherical and converging — Correct. The wavefront becomes spherical and converges to the focal point.

Option (3) Spherical and diverging — Incorrect. A diverging spherical wavefront is produced by a concave lens, which is thinner at the centre.

Option (4) Cylindrical and diverging — Incorrect. A spherical lens gives a spherical wavefront, not a cylindrical one, and it converges the light.

NCERT (2026-27):
Reflection of a Plane Wave by a Plane Surface – “… In Fig. 10.7(b) we consider a plane wave incident on a thin convex lens; the central part of the incident plane wave traverses the thickest portion of the lens and is delayed the most. The emerging wavefront has a depression at the centre and therefore the wavefront becomes spherical and converges to the point F which is known as the focus.”