# Find corresponding wavenumber from FFT - Python

I have a set of data taken from a high speed camera. I've done some image processing which results in getting a pixel location at each frame. This oscillates with time and so I have performed an FFT on the data using scipy.fft.rfft and I worked out the corresponding frequencies using rfftfreq so I can plot amplitude against frequency. However the theoretical data I am comparing to uses wavenumber on the x-axis rather than frequency. Where wavenumber is denoted as $$k=\frac{2\pi}{\lambda}$$

How can I calculate the corresponding wavenumbers (or wavelength) from my FFT data so I can compare my data against the theoretical predictions.? My camera has a frame rate of 27000 frames per second and I recorded for a duration of approximately 4.3 seconds and a total frame count of ~ 116500