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   Converts frequency to wavelength.
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   Form:
   lambda = FrequencyToWavelength( f, c )
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   ------
   Inputs
   ------
   f           (1,:)     Frequency
   c           (1,1)     Propagation speed (default is speed of light)
                        
   -------
   Outputs
   -------
   lambda		  (1,:)     Wavelength
                         
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