The principle of IR spectroscopy. For IR spectroscopy light is cast on a sample of compounds. Depending on its constitution, each compound has a characteristic absorption pattern for the received light. This results in an additive superposition of absorption, which is subsequently recorded as a mixture spectrum. Here, each compound contributes its absorption fingerprint quantitatively to the mixture. The fingerprint can be used for compound identification, because the absorption depends on all paths through the compound that oscillate such that the permanent dipole changes.