Since all parallel rays have travelled the same distance when they hit the surface DD' perpendicular to the rays, all parallel rays have the same phase at their intersection points with the plane DD' ...Since all parallel rays have travelled the same distance when they hit the surface DD' perpendicular to the rays, all parallel rays have the same phase at their intersection points with the plane DD' . If point A is on the interface sought for, the travel time for a ray from D to F via A must be minimum and hence must be the same for all points A.
