Explain the wavefront and its types. 

When we drop a small stone on a calm pool of water, waves spread out from the point of impact. Every point on the surface starts oscillating with time, hence at any instant, the surface would show circular rings on which the disturbance is maximum.

All points on such a circle are oscillating in phase because they are at the same distance from the source. Such a locus of points, which oscillate in phase is called a wavefront. Thus a wavefront is defined as a surface of constant phase.

The speed with which the wavefront moves outwards from the source is called the speed of the wave.

The energy of the wave travels in a direction perpendicular to the wavefront.

Line perpendicular to the wavefront and indicating the direction of propagation of the wave is called ray. Hence, the wavefront and ray are perpendicular.

If a point source emitting waves uniformly in all direction, then the locus of points which have the same amplitude and vibrate in the same phase are spheres (in three dimension) which is known as spherical wave as shown in figure $(a)$ such a waves are diverging.At a large distance from the source, a small portion of the sphere can be considered as a plane. It isknown as a plane wave. It is shown in figure $(b)$.

Wavefronts originating from a linear source and propagating in three dimensional homogenous and isotopic medium are cylindrical wavefront. For example : The waves emanating from the tubelight. It is shown in figure $(c)$.