In three dimensions, the corresponding plane wave term becomes formula_14, which simplifies to formula_15 at a fixed time formula_8, where formula_17 is the position vector of a point in real space and now formula_18 is the wavevector in the three dimensional reciprocal space. The constant formula_19 is the phase of the wavefront (plane of constant phase) through the origin at time formula_8, and formula_21 is a unit vector perpendicular to this wavefront. The wavefronts with phase formula_19 comprise a set of parallel planes, equally spaced by the wavelength formula_11.