## § Eikonal Equation [WIP ]

#### § 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