Although the word "monochromatic" is not exactly accurate, since it refers to light or electromagnetic radiation with well-defined frequency, the spirit is to discover the eigenmode of the wave equation in three dimensions. Following the derivation in the previous section on plane-wave eigenmodes, if we again restrict our solutions to spherical waves that oscillate in time with well-defined constant angular frequency ω, then the transformed function ru (r, t) has simply plane-wave solutions: r u ( r , t ) = A e i ( ω ω t ± ± k r ) , or u ( r , t ) = A r e i ( ω ω t ± ± k r ) .