shape of the wavefront at t = * , we draw spheres of radius v * from each point on the spherical wavefront where v represents the speed of the waves in the medium. If we now draw a common tangent to all these spheres, we obtain the new position of the wavefront at t = * . The new wavefront shown as G G in Fig. .
is again spherical with point O as the centre. The above model has one shortcoming: we also have a backwave which is shown as D D in Fig. . .
Huygens argued that the amplitude of the secondary wavelets is maximum in the forward direction and zero in the backward direction; by making this adhoc assumption, Huygens could explain the absence of the backwave. However, this adhoc assumption is not satisfactory and the absence of the backwave is really justified from more rigorous wave theory. In a similar manner, we can use Huygens principle to determine the shape of the wavefront for a plane wave propagating through a medium (Fig. .
). FIGURE . Huygens geometrical construction for a plane wave propagating to the right. F F is the plane wavefront at t = and G G is the wavefront at a later time * .
The lines A A , B B … etc, are normal to both F F and G G and represent rays.