Write the limitation of Huygen's principle.

(N/A) According to Huygen's principle,every point on a wavefront acts as an independent secondary source and emits small secondary wavelets.
Huygen's principle assumes that the amplitude of these secondary wavelets is maximum in the forward direction and zero in the backward direction.
However,Huygen's could not provide a theoretical justification for why the wave does not propagate backwards; this was an ad-hoc assumption.
Later,scientists such as Voigt and Kirchhoff provided a mathematical explanation,stating that the intensity of the secondary wave is proportional to the factor $(1 + \cos \theta)^2$,where $\theta$ is the angle made by the wavefront with the direction of propagation.
In the forward direction,$\theta = 0^{\circ}$,so the intensity factor is $(1 + \cos 0^{\circ})^2 = (1 + 1)^2 = 4$ (maximum).
In the backward direction,$\theta = 180^{\circ}$ (or $\pi$ radians),so the intensity factor is $(1 + \cos 180^{\circ})^2 = (1 - 1)^2 = 0$. Thus,there is no propagation of waves in the backward direction.