What is the shape of the wavefront of the light emerging from a convex lens when a point source is placed at its focus?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Planar
A wavefront is a surface of constant phase. The energy of the wave travels along the rays, perpendicular to the wavefront. So the shape of the emerging wavefront follows from the directions of the emerging rays.
Option (1) Spherical — Incorrect. A spherical wavefront goes with rays that spread out from a point or close in on a point. That is the wavefront which leaves the point source and reaches the lens. The wavefront that emerges from the lens has a different shape.
Option (2) Cylindrical — Incorrect. The rays normal to a cylindrical surface spread out sideways from its axis. But the rays leaving the lens do not spread at all.
Option (3) Planar — Correct. Rays from a point source at the focus of a convex lens emerge parallel to the principal axis. The surfaces perpendicular to a set of parallel rays are planes, so the emerging light has a plane wavefront.
Option (4) Conical — Incorrect. The rays normal to a conical surface slant towards or away from the axis of the cone. So rays on opposite sides of the cone point in different directions. They are not all parallel.