Since \(2\pi\) represents the number of radians in one cycle and wavelength \(\lambda\) is in meters, the angular wave number \(k\) is measured in radians per meter (rad/m). We use the adjective \(\textit{angular}\) since the term "wave number" is typically used for \(1/\lambda\), the reciprocal of the wavelength. This quantity multiplied by \(2\pi\) is the angular wave number \(k = 2\pi{f}\).
In terms of angular wave number, we can rewrite Equation 13-4 as