eikonal

"Eikonal" is defined as the line of constant optical path length of a wave, which is obtained by multiplying the line of constant phase by λ/2π,whereλis the wave length. The constant plane of eikonal S expresses the plane of the same phase of the wave. The gradient of eikonal, ∇S gives the traveling direction of the wave. |∇S | gives the refractive index at the corresponding position. The concept of eikonal, instead of the refractive index, has been applied to describe the local change of the amplitudes of Bloch waves in a distorted crystal.