According to wave theory, light travels in the form of waves and radiant energy is emitted by fluctuation of electric charge and magnetic field. Infra-red spectroscopy is an important spectroscopy which gives almost complete information about the structure of a compound. The absorption of infrared radiations can be expresses either in terms of wavelength or wave number. Because of absorption of infra-red radiation, molecules become excited and move to lower to higher vibrational level.
If a molecule has a center of symmetry, then the vibrations are Centro symmetric and are inactive in the infra-red region but active in Raman, known as active vibrations.
The vibrations which are not Centro symmetric are active in infra-red region but inactive in Raman, called as forbidden vibrations. Since the absorption of infra-red region is quantized, a molecule of the organic compound will show a number of peaks in the infrared region. Where h = Plank's constantIf the falling radiations are belong to mid-infrared region than the energy would be just equivalent to the molecular vibration energy level. The absorption of radiation can be possible only when the dipole moment of the whole molecule changes due to certain molecular vibrations.
IR spectrum is a record of intensity of the bands as a linear function of T against the corresponding wave number. The Infra-red spectroscopy can be employed to establish the identity of two compounds or to determine the structure of a new compound. The identity of an organic compound can be established by using its finger print region (1400-1900 cm-1). The identity of an organic compound is confirmed of its finger print region exactly matches with the known spectrum of that compound. As compounds contain same functional group may have similar absorptions above 1500 cm-1 but they differ considerably in finger print region. Each functional group absorbs a certain frequency of radiation and show corresponding peak.
All functional groups present in molecule absorbed at their characteristic wave number and the shift due to environment effect can be easily observed in spectrum.
If the IR spectrum of two compounds is exactly same, it shows that they are identical compounds.
Infra red spectroscopy is quite useful for studying the rate and progress of a chemical reaction. With the progress of reaction the band of >C=O group diminished and band of -O-H group at 3300 cm-1 appears.

The progress of reaction can be determined by using the rate of disappearance of a certain absorption band of the reactant molecule and the rate of appearance of the absorption band of the product group.
Infra red spectroscopy is also useful in the detection of the impurity in a compound by comparing its spectrum with the spectrum of the authentic sample of the compound.
A pure sample always consists of sharp peaks and bands while the impure sample will have poor bands with some additional bands. Quantitative analysisThe quantitative analysis of an organic mixture can be done by measuring the intensities of the absorption bands of each component and by knowing the optical density of the absorption bands for the pure substance.
The presence of certain bands decide the isomer of Xylene and the optical density values gives information about the percentage composition of each isomer in given sample.
Distinction between two type of hydrogen bondingInfra red spectroscopy can be detect the presence of H-bonding as well as the type of hydrogen bonding; Intermolecular or Intramolecular.
Study of complex formation Infra redInfra red spectroscopy is useful in prediction of structure of complex molecules like penicillin .
Infrared spectroscopy is useful in prediction of unknown compound and presence of certain functional groups.
Conformation analysis IR technique is useful in determination of relative stability of various conformations of cyclic compounds. Geometrical IsomerismOnly those vibrations are IR active which involves the change in dipole moment of the molecule and the intensity of absorption bands depends upon the change in dipole moment of molecule.
The main difference in the highlighted carbon atom of both molecule  and carbocation is the hybridization.
The infra-red spectrum is containing a large number of absorption bands which provides much information about the molecule's structure. Hence molecules in the ground state of vibration absorb this radiation and transit to the first excitation state of vibration. The most common source of IR spectrometer is Nernst glower which consists of a rod of a sintered mixture of the oxides of Zirconium, Ytterbium and Erbium. Instead of thermopile detector, other detectors are also available, like Pyroelectric detectors which are made from a single crystalline wafer of a pyroelectric material like triglycerine sulphate. The region from 1450 to 600 cm-1 is called as fingerprint region and the absorption bands in the 4000 to 1450 cm-1 region are usually because of stretching vibrations of diatomic units, and known as the group frequency region. This technique is quite useful to predict the presence of certain functional groups which absorb at definite frequencies.
Hence the position of peak gives the information regarding the presence of certain functional group. Ur technique plays an important role in crime investigation by locating the criminal offenders.

Like both conformations of 1, 2-Dichloroethane show two bands at 1235 cm-1 and 1291 cm-1 in IR spectrum.
Since there is a change in dipole moment in case of cis-isomer whereas trans-isomers are non-polar in nature, therefore both geometrical isomers show different bands in IR spectrum. These are radiations produced by the oscillation of electric charge and magnetic field residing on the atom. Hence the absorption of infra-red caused to effect vibrational as well as rotational level and also called as vibrational- rotational spectra.
However, it is not necessary that absorption takes place with the given energy of mid-infra red region. When an electric field is applied across detector, electric polarization occurs which persist after removing the field also. There are 18 infra-red active C-C and CH2 rocking and twisting vibrations for boat conformation while chair conformation show only 5 absorption bands in the region of 1350-700 cm-1.
Due to more stability, the staggered form predominant at lower temperature whereas gauche form dominates at high temperature. The absorption of infrared radiations is possible by only those bonds of molecule which can show a change in dipole moment by the absorption of radiations. For example, when hydroxy groups get associate like in solid or liquid state, the absorption band shifted towards lower frequency.
Such types of vibrational transitions which involve the change in dipole moment are known as infra-red transitions.However those vibrational transitions which are not accompanied by the change in dipole moment are not directly observed and called as infra-red inactive transition.
Another type of detectors is Photoelectric detectors like mercury cadmium telluride detector which consist a film of semiconducting material deposited on a glass surface, sealed in an evacuated envelope. The sequence of all types of electromagnetic radiations in order of their increasing wavelength or decreasing frequencies is known as complete electromagnetic spectrum.The interaction between matter and electromagnetic radiation is defined as spectroscopy. The lowering in the absorption band of >C=O (straching) in the enolic form is because of intra-molecular hydrogen bonding which established by resonance.
When a light radiation passed through a sample of organic compound, some of the wavelengths are absorbed. The absorption of energy changes the vibration, rotational and electronic levels of molecule.

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