# Translations:Wavefronts and raypaths/1/en

A *homogeneous* medium is a material with properties independent of position. An *isotropic* medium is a material with properties independent of direction of travel. An *anisotropic* medium is a material in which a physical property (such as velocity) depends on the direction. A homogeneous and isotropic medium is a material with properties independent of both position and direction of travel. The simplest type of wave propagation can be illustrated by a disturbance originating at a point in a homogeneous and isotropic medium. In particular, the velocity is constant and independent of direction everywhere in the medium. In Figure 2, *S* is the location of the point source of the disturbance. The source itself could be a shot of dynamite, for instance. The wave proceeds outward in all directions from the source, and its successive positions will assume the form of expanding concentric spheres with *S* as center. The successive positions of the wave after equal intervals of elapsed time will be equally spaced spheres - spheres whose radii differ by equal increments. In Figure 2, the fifth wavefront, which represents the locus of the disturbance after five time intervals of equal length, is the spherical arc *ABC*. If the total elapsed time is *t* and the velocity is *v*, then the radius* SB* is equal to *vt*.