title: How to find corresponding wavenumber of FFT in Python
body: <p>I have some data which I have performed an FFT on using <code>scipy.fft.rfft</code> and I found the corresponding frequencies using <code>scipy.fft.rfftfreqs</code></p>
  <p>Is there a way to find the corresponding wavenumbers or wavelengths rather than frequencies? Since I don't know what the velocity is I am not sure how to convert frequencies into wavelength and ultimately wavenumber.</p>
  <p>I would like to compare my data with theoretical data which is in terms of wavenumber. My data is high speed imaging taken at 27000 frames per second for 4.3 seconds. I perform image analysis to track a certain position over time. This gives me a data set of pixel values which vary over time which I performed the FFT on.</p>