Electromagnetic (EM) radiation is a periodically changing or oscillating electric field propagating in a certain direction with a magnetic field oscillating at the same frequency but perpendicular to the electric field.
Figure 1. A schematic representation of EM radiation. The wavelength is represented by λ.
EM radiation may be considered as a traveling wave or as a stream of massless elementary particles, often called photons. As a wave, it can be characterized by its wavelength λ (the length of one wave), its frequency ν (the number of vibrations per unit time) and its wavenumber k (the number of waves per unit length
Interaction of Light and Molecules
Roughly, there are 3 possible effects of interaction between radiation and molecules. These are scattering, absorption, and emission. Absorption is the process by which the energy of a photon is taken up by the matter, and this process plays a key role in IR spectroscopy. There are several types of physical processes that could lie behind absorption, depending on the quantum energy of the particular frequency of EM radiation. For example, high energetic ultraviolet (UV) radiation can cause ionization and visible light usually causes electron transitions. As told before, the energy levels for all physical processes at the atomic and molecular levels are quantized, and if there are no available quantized energy levels with spacings which match the quantum energy of the incident radiation, then the material will be transparent to that radiation. In view of IR spectroscopy, I will now focus on the absorption of IR radiation by matter.