To get the wave number $k$, you need to know the wavelength $\lambda$ of the wave. Step 1: Identify the wavelength ($\lambda$). The wavelength is the spatial period of the wave, the distance over which the wave's shape repeats. It is usually given in meters (m). Step 2: Use the formula for wave number. The wave number $k$ is defined as: $$k = \frac{2\pi}{\lambda}$$ where: • $k$ is the wave number (in radians per meter, $\text{rad/m}$ or $\text{m}^{-1}$) • $\pi$ is the mathematical constant pi (approximately 3.14159) • $\lambda$ is the wavelength (in meters, m) Example: If a wave has a wavelength of $\lambda = 0.5 \, \text{m}$, then its wave number would be: $$k = \frac{2\pi}{0.5 \, \text{m}} = 4\pi \, \text{rad/m}$$ $$k \approx 12.57 \, \text{rad/m}$$