In the formulation of Hyugens principle it is said that the secondary waves are spheres but on the plots for determination of the wavefront there suddenly become bemispheres? What is the reason for this mismatch? Is there any physical explanation based on momentum of incoming wave in sense that it influences the atoms of the media? What media do one has in case of EM waves? Maybe Hyugens does not have any physical meaning but is just an observational geometrical rule?

In the formulation of Huygens principle, it is said that the secondary waves are spheres but on the plots for determination of the wavefront there suddenly become hemispheres? What is the reason for this mismatch? Is there any physical explanation based on the momentum of the incoming wave in the sense that it influences the atoms of the media? What media does one have in case of EM waves? Maybe Huygens does not have any physical meaning but is just an observational geometrical rule?