ABSTRACT

We first define the notion of a wavefront. The wavefront is a surface over which an optical disturbance has a constant phase. A plane wave has a planar wavefront perpendicular to the direction of propagation of the wave, while a spherical wave has a wavefront in the shape of a spherical surface. The Huygens-Fresnel principle states that every point on a primary wavefront serves as the source of spherical secondary wavelets such that the wavefront at some later time is the envelope of these wavelets. According to Fresnel these secondary wavelets can interfere. The wavelets advance with a speed and frequency equal to that of the primary wave at each point in space. Fig. 3.1, for example, explains the refraction of light using the Huygens principle.