2) Now, according to Huygens principle, each point of the
wavefront is the source of a secondary disturbance and the wavelets
emanating from these points spread out in all directions with the speed
of the wave These wavelets emanating from the wavefront are usually
referred to as secondary wavelets and if we draw a common tangent
to all these spheres, we obtain the new position of the wavefront at a
later time FIGURE 10