Wave-front :

The locus of all the particles of the medium vibrating in the same phase at a given instant is called wave-front.

(a) In case of light diverging from a point source, the shape of wave-front is a circle

Figure of wave-front formed by a point source

(b) In case of light emerging out of a convex lens when a point source is placed at its focus, the wave-front is a parallel grid.

Fig: Light rays passing through a convex lens

Figure of wave-front formed by a convex lens when point source is at its focus

(c) When light source is a distance star intercepted by earth, the wave-front formed is a plane