code: def validate_wave_unit(wave_unit):
      
      output_unit = validate_unit(wave_unit)
      unit_type = output_unit.physical_type
  
      if unit_type not in ('length', 'wavenumber', 'frequency'):
          raise exceptions.SynphotError(
              'wavelength physical type is not length, wave number, or '
              'frequency: {0}'.format(unit_type))
  
      return output_unit
docs: Like :func:`validate_unit` but specific to wavelength.