# Dictionary:Eikonal equation

(ī kōn’ ∂l) A form of the wave equation for harmonic waves in which the local velocity *V* is compared to a reference velocity *V*_{R} (analogous to comparing a velocity to the speed of light in vacuum):

where *n* is an index of refraction and is the wave function. Valid only where the variation of properties is small within a wavelength, sometimes called the ‘‘high-frequency condition.’’