A wavefront is the continuous locus of vibrating particles of the medium which are in the same state of vibration or phase. If a point source in n isotropic medium (i.e., a medium in which the wave travels with the same speed in all directions) is sending out waves in three dimensions, the wavefronts are spheres centered on the source as shown in figure (i), such a wavefront is called a spherical wavefront. At a large distance from a source of any kind, the wavefront will appear plane as shown in figure (ii), such a wavefront is called the plane wavefront.