# Translations:Ray theory/19/en

The concept of a ray is extremely useful. Rays are curves drawn in space, and they correspond to the directions of flow of propagated energy. In other word, rays are flow lines. Being flow lines, rays cannot cross each other. As such, the ray is a mathematical device rather than a physical entity. In practice, we can produce very narrow beams or pencils (for example, a laser beam), and we might imagine a ray to be the unattainable narrowness limit of such a beam. We recall that an isotropic medium is a substance for which each physical property at any point has the same value when measured in different directions. An isotropic medium can be either homogeneous (i.e., consisting of points all of the same kind) or inhomogeneous (not homogeneous). In an isotropic medium, rays constitute orthogonal trajectories of the wavefronts, that is to say, such rays are lines normal to the wavefronts at all points of intersection. In such a medium, a ray is evidently parallel to the propagation vector. However, this ceases to be the case in anisotropic materials (whose properties vary as a function of direction).