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