Write the limitation of Huygen's principle.

Explanation

According to Huygen.s principle every particles of a wavefront behaves as an independent secondary source and emits by itself small secondary waves.
The amplitude of such secondary wavelets is maximum in the forward direction and zero in the backward direction
By making such adhoc assumption, Huygen.s could not explain why the wave was not propagating backwards. However, this adhoc assumption is not satisfactory.
Scientist such as Voigt and Kirchhoff explain such a limitation and state that the intensity of the secondary wave proportional to the term `1+cos^(2)((theta)/(2))` where `theta` made by wavefront to the direction of propagation.
In the direction of propagation `theta=0^(@)`, hence the intensity of light becomes `cos^(2)""(theta)/(2)=1+cos0^(@)=2` in backward direction `theta=pi`, hence intensity
`cos^(2)""(theta)/(2)=1+cos{2((theta)/(2))}=1+costheta=1+(-1)=0`
Hence, the intensity of light in backward direction of wave propagation is zero. So there is no propagation of wave backward.