# Huygens' principle and phase shift

I wonder why the amplitude of Huygens' wavelet is equal to the amplitude of incident wave multimpled by $$i / \lambda$$. I found this formula in 'Modern Optics' by Guenther p. 335 unfortunately without proof. Has anyone seen the derivation of this fact.

Could you recomend me some good books in optics about wave nature of light.