Define the angular wave number and provide its mathematical equation.

(N/A) The angular wave number,also known as the propagation constant,is defined as the phase change per unit distance of a wave.
It represents the number of radians of phase change per unit length.
Mathematically,the angular wave number $k$ is given by:
$k = \frac{2\pi}{\lambda}$
where $\lambda$ is the wavelength of the wave.
The $SI$ unit of angular wave number is $\text{rad/m}$ or $\text{m}^{-1}$.