§ 1D

§ nD

• Since everything is determined by 1D parametrization (curves)
• $|\nabla \phi(r)| = 1/v(r)$
• $r$ is position in space.
• $v(r)$ is the speed of light at position $r$.
• $\phi(r)$ represents the travel time of a wavefront to reach $r$.
• So, the gradient of the travel time is

§ References

• Video on eikonal equation