Fresnel-Huygens Principle Part-1| Lad trying to understand light

Just trying to understand the working of fresnel-huygen principle...
References: Born, M., Wolf, E., Bhatia, A., Clemmow, P., Gabor, D., Stokes, A., . . . Wilcock, W. (1999). Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light (7th ed.). Cambridge: Cambridge University Press. doi:10.1017/CBO9781139644181