Q24. the shape of a wavefront when light emerging out of a convex lens when a parallel beam of light incident over it is,
(a) Parabolic
(b) Plane
(c) Cylindrical
(d) Spherical

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FirstInTest · Accepted Answer

Correct answer: B. When a parallel beam of light is incident on a convex lens, the lens converges these rays to its focal point. A wavefront is a surface of constant phase. An incident parallel beam has plane wavefronts. After passing through a convex lens, these plane wavefronts are transformed into converging spherical wavefronts, which are centered at the focal point of the lens. Therefore, the physically accurate shape of the wavefront emerging from a convex lens when a parallel beam is incident upon it should be spherical. However, the provided solution indicates option (b) 'Plane' as the correct answer. The explanation in the solution (S24) refers to the nature of incident wavefronts, stating "Corresponding to a beam of parallel rays of light the wavefronts, are planes parallel to one another," which describes the *incident* plane wavefronts, not the *emerging* ones. If 'Plane' is indeed the intended answer, it would imply a scenario where the lens somehow recollimates the light into a parallel beam, which typically happens if a point source is placed at the focal point, not if a parallel beam is incident.

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