| Type of Radiation | Frequency Range (Hz) | Wavelength Range | Wave Number [cm−1] | Type of Transition |
| Mid infrared | 1013 - 1014 | 25 µm - 2.5 µm | 400 - 4000 | Molecular vibrations |
| Far infrared Millimeter waves | 3 × 1011 - 1013 | 1 mm - 25 µm | 10 - 400 | Molecular rotations, electron spin flips |
| Microwaves Radio waves | <3 × 1011 | >1 mm | <10 | Nuclear spin flips |