To clarify and extend the description of units for the wavenumbers of infrared radiation, it is necessary to know that the wavelength of any electromagnetic radiation [infrared radiation, visible light and ultraviolet radiation are three familiar examples] is related to its frequency by the equation:

l = c/n

l is the symbol used for wavelength, c is the speed of light and n is the frequency of the radiation. The wavenumber is a useful unit because the numbers are manageable in the infrared region.

The vibrators [H₂O, N₂O and O₃] are also capable of being encouraged to rotate by interacting [absorbing] infrared radiation. This is because they satisfy another of the quantum rules that specifies that such molecules should possess a permanent dipole moment. Those three molecules are the rotators.

CO₂ is a linear molecule and cannot be induced to rotate by interaction with radiation; its vibrational motion can be caused by the absorption of infrared radiation only because the bending mode gives the molecule a transient dipole.

CH₄ is a regular tetrahedral molecule and does not have a permanent dipole moment; it is not a rotator, but again it does have a vibrational mode which allows it to have a transient dipole moment.