When setting a new value for this attribute, the new value can be
supplied either directly as a frequency, or indirectly as a wavelength
or energy, in which case the supplied value is converted to a frequency
before being stored. The nature of the supplied value is indicated by
appending text to the end of the numerical value indicating the units in
which the value is supplied. If the units are not specified, then the
supplied value is assumed to be a frequency in units of GHz. If the
supplied unit is a unit of frequency, the supplied value is assumed to
be a frequency in the given units. If the supplied unit is a unit of
length, the supplied value is assumed to be a (vacuum) wavelength. If
the supplied unit is a unit of energy, the supplied value is assumed to
be an energy. For instance, the following strings all result in
a rest frequency of around 1.4E14 Hz being used: "1.4E5", "1.4E14 Hz",
"1.4E14 s-1", "1.4E5 GHz", "2.14E-6 m", "21400 Angstrom", "9.28E-20 J",
"9.28E-13 erg", "0.58 eV", etc.
When getting the value of this attribute, the returned value is always a frequency in units of GHz.