displacement is of the order of a few millimetres, which is much larger than the wavelength of light. Hence the candle cannot be seen. If we are able to move one of the cardboards by a micrometer or less, light will be able to diffract, and the candle will still be seen. One could add to the first sentence in this box: It learns how to bend as it grows up !
FIGURE . (a) A diverging spherical wave emanating from a point source. The wavefronts are spherical. small portion of the sphere can be considered as a plane and we have what is known as a plane wave [Fig.
. (b)]. Now, if we know the shape of the wavefront at t = , then Huygens principle allows us to determine the shape of the wavefront at a later time * . Thus, Huygens principle is essentially a geometrical construction, which given the shape of the wafefront at any time allows us to determine the shape of the wavefront at a later time.
Let us consider a diverging wave and let F F represent a portion of the spherical wavefront at t = (Fig. . ). 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 . (b) At a large distance from the source, a small portion of the spherical wave can be approximated by a plane wave. FIGURE .
F F represents the spherical wavefront (with O as centre) at t = . The envelope of the secondary wavelets emanating from F F produces the forward moving wavefront G G . The backwave D D does not exist. Thus, if we wish to determine the