Wavefronts and Huygens' Principle

A wavefront is defined as the locus of all the points in a medium that are vibrating in the same phase at a given instant of time -- every point lying on one wavefront is, at that instant, at the identical stage of its oscillation as every other point on the same wavefront. The shape of the wavefront is fixed by the shape of the source and the distance travelled: a point source close to the observer sends out spherical wavefronts, expanding concentric spheres centred on the source; a long linear source (a narrow illuminated slit, for instance) sends out cylindrical wavefronts; and at a large distance from any source, or after the light has been rendered parallel by a converging lens or a concave mirror, a small enough patch of any wavefront can be treated as a plane wavefront, a flat sheet advancing perpendicular to itself. A ray, in this picture, is simply a line drawn perpendicular to the wavefront at any point, showing the local direction in which the wave -- and the energy it carries -- is advancing; the familiar ray diagrams of geometrical optics are nothing but a bundle of such perpendiculars to an underlying wavefront. …