Abstract Infrared (IR) spectra originate from transitions between vibrational energy levels of vibrating atomic groups and they are usually observed as absorption spectra. Molecules, atomic groups and even the whole lattice in crystals may interact with the electromagnetic field of light and absorb part of the energy carried by the light. The molecule interacts only with light that carries the right amount of energy to promote the molecule from one discrete energy level (ground state) to another. When it happens that IR radiation can be absorbed and a ground state molecule can be promoted to its first excited vibrational state, we say that the molecule has made a transition between the ground state and the excited state. Light of IR frequencies can generally excite molecules or functional atomic groups from one vibrational energy level to another. Hence, IR spectroscopy is called “vibrational spectroscopy”. Visible and UV radiation are higher energetic and can promote the redistribution of electrons in a molecule or atomic group, such that the electronic potential energy of the molecule is changed (“electronic spectroscopy”). Theoretical background Bohr’s frequency condition Using the terminology of quantum mechanics, we have to state that if a molecule is placed in an electromagnetic field (IR light), a transfer of energy from the field to the molecule will occur only when Bohr’s frequency condition is satisfied: