If ℓ is the component of r in the direction of k ‍, ‍ the field (5) can be written E k e i ( k ℓ ℓ − − ω ω t ) . If the argument of e i ( ⋯ ⋯ ) is to be constant, ℓ must increase at the velocity ‍ ω ω / k , known as the phase velocity. This in turn is equal to c / n , where c is the phase velocity in the reference medium (taken as vacuum) and n is the local refractive index w.r.t. the reference medium. Solving for k gives ‍ k = n ω ω / c , i.e.