موجة كهرومغناطيسية
The plane wave spectrum arises naturally as the eigenfunction or "natural mode" solution to the homogeneous electromagnetic wave equation in rectangular coordinates (see also Electromagnetic radiation, which derives the wave equation from Maxwell's equations in source-free media, or Scott [1998]). In the frequency domain, with an assumed time convention of formula_69, the homogeneous electromagnetic wave equation is known as the Helmholtz equation and takes the form: