Wavefronts

The surface over which particles are vibrating in the same phase. The surface is normal to rays in isotropic media.
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Isotropic: A body is isotropic if its properties are the same in all direction

       Consider a point source of light as S ( Figure given below ). Waves emitted from this source will propagate outwards in all directions with speed c (c is the speed of light). After time t, they will reach the surface of a sphere with center as S and radius ct. Every point on the surface of this sphere will be set into vibration by the waves reaching there. As the distance of all these points from the source is the same, so their state of vibration will be identical. In other words we can say that all the points on the surface of the sphere will have the same phase.

Phase: Particles in periodic motion due to the passage of a wave are said to be in the same phase of vibration if they are moving in the same direction with the same relative displacement. Particles in a wavefront are in the same phase of vibration and the distance between the phases are the same is the wavelength i.e λ.

   Fig  . Spherical wavefronts

Such a surface on which all the points have the same phase of vibration is known as wavefronts.

         Thus in case of a point source, the wavefront is spherical in shape. A line normal to the wavefront including the direction of motion is called a ray of light.

          With time, the wave moves farther giving rise to new wave fronts. All these wavefronts will be concentric spheres of increasing radii as shown in the figure given above. Thus the wave propagates in space by the motion of the wavefronts is one wavelenth. It can be seen that as we move away at greater distance from the source, the wavefronts are parts of spheres of very large radii. A limited region taken on such a wavefront can be regarded as a plane wavefront ( Shown in figure given below ). For example, light from the sun reaches the Earth in plane wavefronts.

   Fig  . Plane wavefronts

      
       In the study of interference and diffraction, plane waves and plane wavefronts are considered. A usual way to obtain a plane wave is to place point source of light at the focus of a convex lens. The rays coming out of the lens will constitute plane waves.