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Question
What is wavefront? How is it related to rays of light? What is the shape of the wavefront at a point far away from the source of light?
Solution
The locus of all points having the same phase at a given instant of time is called a wavefront.
Consider a light source O in a homogeneous isotropic medium with a speed of light of v. Light is emitted from the source in all directions. The disturbance (light energy) from the source covers a distance vt in all directions in time t, i.e., it reaches all points at a distance vt from the point source. The surface of a sphere with centre O and radius vt is the locus of these points in the same phase. Three dimensional waves like the sound waves produced by a source of sound, or light waves produced by a light source, travel in all directions away from the source and propagate in three dimensions. Such a wave is called a spherical wave. The arrows are perpendicular to the spherical surfaces and show the direction of propagation of the waves. These arrows are the rays of light that we have considered in the earlier study of optics. consider a spherical wavefront that has travelled a large distance away from the source. If we take a small portion of this wavefront, it will appear to be a plane surface (just like the surface of the earth around us appears to be flat to us) with the direction of propagation perpendicular to it. In such a case the wave is called a plane wave.
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