Working Principle of Infra-Red (IR) Spectroscopy

Infra-Red (IR) radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. Electromagnetic radiation consists of an oscillating electrical field and an oscillating magnetic field perpendicular to each other. The alternating electrical field of the radiation interacts with fluctuations in the dipole moment of the molecule. If the frequency of IR radiation matches the vibrational frequency of the molecule, then radiation will be absorbed, causing the change in amplitude of molecular vibration.


A beam of infrared light is produced and split into two separate beams. One is passed through the sample, the other is passed through the reference, which is often the substance the sample is dissolved in. The beams are reflected back towards a detector. The two signals are compared and obtained as graph by chart recorder.

Uses of IR spectroscopy
1. Techniques have been developed to access the quality of tea leaves using IR spectroscopy
2. Characterization of thin films, powders, polymers, minerals, solution and many more
3. Following the progress of reaction through time