(a) State Huygens principle. How did Huygens justify the absence of the backwave on a spherical wavefront?

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.

The amplitude of the secondary wavelets is maximum in the forward direction and zero in the backward direction.

Alternatively:

Amplitude $\displaystyle (\mathrm{A}) \propto(1+\cos \theta)$; for back wave $\displaystyle \theta=180^{\circ} \Rightarrow$ Amplitude $\displaystyle =0$

The linear width of the central maximum will decrease.