where is the (constant) complex amplitude vector,  is the imaginary unit,  is the wave vector (whose magnitude is the angular wavenumber),  is the position vector,  "ω" is the angular frequency,  is time, and it is understood that the "real part" of the expression is the physical field. The magnetizing field has the same form with the same and "ω". The value of the expression is unchanged if the position varies in a direction normal to ; hence "is normal to the wavefronts".