What is the shape of a wavefront when light emerging out of a convex lens when a parallel beam of light incident over it ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Spherical
A wavefront is a surface of constant phase, and the rays are always perpendicular to it. A parallel beam of light has a plane wavefront.
A convex lens slows down the central part of the plane wavefront more, because the lens is thickest there. The emerging wavefront is curved and its rays all meet at the focus, so it is a converging spherical wavefront centred on the focal point.
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.”