# Translations:Ray theory/21/en

Within homogeneous isotropic materials, rays are straight lines. By symmetry, they cannot bend in any preferred direction because no such preferred direction exists. Moreover, because the propagation speed is identical in all directions, the spatial separation between two wavefronts, measured along rays, must be the same everywhere. Points at which a single ray intersects a set of wavefronts are called corresponding points, as for example points A, B, and C in Figure 15. Evidently the separation in time between any two corresponding points on any two sequential wavefronts is identical. In other words, if wavefront ${\displaystyle S_{\rm {l}}}$ transforms into wavefront ${\displaystyle S_{\rm {2}}}$ after a time ${\displaystyle \Delta t}$, the distance between corresponding points on any ray will be traversed in the same time ${\displaystyle \Delta t}$. This is true even if the wavefronts travel from one homogeneous isotropic medium into another, and it simply means that every point on ${\displaystyle S_{\rm {1}}}$ can be imagined to follow the path of a ray that arrives at ${\displaystyle S_{\rm {2}}}$ in the time ${\displaystyle \Delta t}$.