(a) A wavefront is defined as a surface of constant phase.
[Alternatively: A wavefront is the locus of all points in the medium that have the same phase.Difference from a ray:
(i) The ray, at each point of a wavefront, is normal to the wavefront at that point.
(ii) The ray indicates the direction of propagation
of wave while the wavefront is the surface of constant phase.
(b) The shape of the wavefront, in the three cases, are as shown.