A plane wave passes through a convex lens. What is the geometrical shape of the emerging wavefront?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → converging spherical
Option (1) planar — Incorrect. A wavefront stays plane only when every part of it is delayed equally, as in a glass slab of uniform thickness. A convex lens delays the centre more than the edges.
Option (2) diverging spherical — Incorrect. A diverging spherical wavefront comes out of a concave lens, which is thinnest at the centre.
Option (3) converging spherical — Correct. Light travels slower in glass, and a convex lens is thickest at the centre, so the middle of the plane wavefront is delayed the most. The emerging wavefront is curved inward and closes in on the focus.
NCERT (2026-27):
Refraction at a Rarer Medium – “… 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.”
Fig. 10.7: Refraction of a plane wave by (b) a convex lens.