The contribution of the wavelets lying on the back of the wave front is zero because of something known as the Obliquity Factor. It is assumed that the amplitude of the secondary wavelets is not independent of the direction of propagation, Sources: byju's.com
According to the Voigt and Kirchhoff's law for wavelets, the contribution of a wavelet in any direction making an angle x with the normal to the wavelet is proportional to {1+cos(x)/2)}
For wavelet lying on the back of secondary wavefront,
x = 180°
→ So, the proportional factor:
→ {1+cos(180°)/2}
→ (1-1/2)
→ (0/2)
→ 0
That's why the contribution of the wavelets lying on the back of secondary wave front is zero
