Figure 4.3 — Plane waves and evanescent waves Figure 4.3 — Plane waves and evanescent waves. (a) The transverse wavenumbers km,x and km,y of plane waves are restricted to the shaded circular area identified by the condition km,x2 + km,y2 ≤ km2, whereas evanescent waves fill the space outside. (b) A plane wave propagating along the z-axis does not present any oscillations in the xy-plane. (c) The spatial frequency in the xy-plane increases as the angle between the propagation direction identified by km and the z-axis increases. (d) The largest number of oscillations in the xy-plane corresponds to a plane wave propagating perpendicular to z.