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