Translations:Two orthogonal sets of circles/1/en

We now must show that the rays and wavefronts always intersect at right angles (Figure 18). The wavefronts are arcs of circles. By the eikonal equation, we know that the wavefront circles are orthogonal to the raypaths. Let H be the point of intersection of a raypath (characterized by initial angle ${\theta }_{\rm {1}}$) and a wavefront (characterized by traveltime t). The center and radius of the raypath circle are, respectively,