Question

In class, while teaching Huygens principle we were taught that a tertiary wavelet exist inner to the original wavefront. But while reading the text I got a notion that backwave doesn't exist. So does it really exist? If not, why doesn't it exist?

Answer 1

The back wave doesn't exist. Huygens argued that the amplitude of the secondary wavelets is maximum in forward direction and zero in the backward direction; by making this adhoc "assumption", Huygens could explain the absence of backwave. However, this adhoc assumption is not satisfactory and the absence of the backwave is really justified from more rigorous wave theory.


Consider a point source ‘S’. Light emitted by ‘S’ travels in all directions. Let AB be a section of the position of wave-front, at any instant, of time ‘t’ as shown in figure. The secondary disturbances emitted by all the points on AB are shown by dotted circles. A1B1 and A2B2 are the two tangent envelopes touching all these secondary disturbances.

According to Stoke’s law, the intensity at each point of the secondary wave is proportional to (1 + cosθ). Here ‘θ’ is angle between normal and central line.

For backward wave-front ‘A2B2’ (shown dotted), θ = π

So, cosθ = cosπ = -1

Thus, intensity at any point on backward wave = k (1+cosπ) = k (1-1) = 0

Therefore back wave-front does not exist.