Q.Distinguish between a wavefront and a ray. What is the shape of the wavefront

A wavefront is the locus of all points of a medium that are vibrating in the same phase at a given instant -- it is a surface (or, in a 2-D diagram, a curve), not a line. A ray, by contrast, is a straight line drawn perpendicular to the wavefront at any point, indicating the local direction of propagation of the wave (and of the energy it carries); a bundle of such perpendiculars to successive wavefronts reproduces the usual ray diagrams of geometrical optics.

1. Close to a point source, the source radiates in all directions equally, so the surfaces of constant phase are spheres centred on the source -- the wavefront there is spherical.
2. At a very large distance from a point source, only a small patch of this (still spherical, but now very large-radius) wavefront is considered, and over that small patch the curvature is negligible enough that the wavefront is treated as a plane wavefront. The same plane-wavefront approximation results immediately after light diverging from a point placed at the focus of a converging lens (or concave mirror) emerges from it, since the lens renders the rays parallel.
   ✓Final answer

   Wavefront = surface of constant phase (not a line); ray = perpendicular to it, showing propagation direction. (a) Spherical close to a point source.

   (b) Plane far from a point source, or after a converging lens.

Recall the formal definitions of wavefront (surface) and ray (perpendicular line), then apply the near/far-field and lens-collimation facts.

• Describing a wavefront as a 'ray of light travelling outward' -- these are two different geometric objects.
• Saying the wavefront becomes plane only 'at infinity' -- in practice it is treated as effectively plane at any sufficiently large but finite distance.