Translations:The eikonal equation - book/9/en

Eikonal equation 14 says that at any point, the gradient of the traveltime is equal to the slowness n times the unit tangent to the ray. Therefore, the gradient and the tangent go in the same direction. Because the gradient is orthogonal to the wavefront and the tangent is along the ray, it follows that the ray is orthogonal to the wavefront: