You must be able to convert energy and frequency, but often spectroscopists are
more interested in working with numbers which span a reasonable range,
rather than always working in energy or frequency.
In IR and Raman, the spectroscopic transition energies are generally
quoted in wavenumber units.
Wavenumbers are the inverse of the wavelength 1/.
Because of the energies involved, it is generally most convenient
to quote typical IR spectroscopic transitions in cm
rather than m.
When a beam of light is passed through a transparent substance, a
small amount of the radiation energy is scattered, the scattering
persisting even if all dust particles or other extraneous matter are
rigorously excluded from the substance. If monochromatic radiation
is used the scattered energy will consist almost entirely of radiation
of the incident frequency.