Huygens wave theory of light failed to explain ?
Answers
As we have seen, Huygens considered that light was propagated in longitudinal waves through a material called the aether. We will now look at his ideas more closely.
Huygens published his theory in 1690, having compared the behaviour of light not with that of water waves but with that of sound. Sound cannot travel through a vacuum but light does, and so Huygens proposed that the aether must fill all space, be transparent and of zero inertia. Clearly a very strange material!
Even at the beginning of the twentieth century, however, scientists were convinced of the existence of the aether. One book states 'whatever we consider the aether to be there can be no doubt of its existence'.
We now consider how Huygens thought the waves moved from place to place.
Consider a wavefront initially at position W, and assume that every point on that wavefront acts as a source of secondary wavelets. (Figure 1 shows some of these secondary sources). The new wavefront W1 is formed by the envelope of these secondary wavelets since they will all have moved forward the same distance in a time t (Figure 1).
There are however at least two problems with this idea and these led Newton and others to reject it:
(a) the secondary waves are propagated in the forward direction only, and
(b) they are assumed to destroy each other except where they form the new wavefront.
Newton wrote: 'If light consists of undulations in an elastic medium it should diverge in every direction from each new centre of disturbance, and so, like sound, bend round all obstacles and obliterate all shadow.' Newton did not know that in fact light does do this, but the effects are exceedingly small due to the very short wavelength of light.
Huygens' theory also failed to explain the rectilinear propagation of light.
The reflection of a plane wavefront by a plane mirror is shown in Figure 2. Notice the initial position of the wavefront (AB), the secondary wavelets and the final position of the wavefront (CD). Notice that he shape of the wavefront is not affected by reflection at a plane surface.
The lines below the mirror show the position that the wavefront would have reached if the mirror had not been there.
We will now show how Huygens' wave theory can be used to explain reflection and refraction and the laws governing them.