# Translations:The wave equation/7/en

Points at which a single ray intersects a set of wavefronts are called *corresponding points*. Evidently, the separation in time between any two corresponding points on any two sequential wavefronts is identical. In other words, if wavefront *S* transforms into after a time , the distance between corresponding points on any and all rays will be traversed in that same time . This obviously will be true even if the wavefronts pass from one homogeneous isotropic medium into another. This simply means that each point on *S* can be imagined as following the path of a ray to arrive at in the same time . Within a homogeneous isotropic medium, the velocity of propagation is identical in all directions. In such a medium, the spatial separation between two wavefronts, measured along a ray, must be the same everywhere.