Eikonal equation
An eikonal equation (from Greek εἰκών, image) is a non-linear first-order partial differential equation that is encountered in problems of wave propagation. The classical eikonal equation in geometric optics is a differential equation of the form where x {\displaystyle x} lies in an open subset of R n {\displaystyle \mathbb {R} ^{n}} , n ( x ) {\displaystyle n(x)} is a positive function, ∇ {\displaystyle \nabla } denotes the gradient, and | ⋅ | {\displaystyle |\cdot |} is the Euclidean norm.