Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer.
The IR region is divided in to three regions: the near, mid, and far IR. The mid IR region is of greatest practical use to the organic chemist. This is the region of wavelengths between 3x10-4 and 3x10-3 cm. Chemists prefer to work with numbers which are easy to write; therefore IR spectra are sometimes reported in m, although another unit, (nu bar or wavenumber), is currently preferred.
A wave number, (cm-1) is the inverse of the wavelength, (cm):