Infrared spectroscopy deals with the recording of the absorption of radiations in the infrared region of the electromagnetic spectrum. The position of a given infrared absorption is expressed in terms of wavelength in micron μ or more commonly in terms of wavenumber (cm−1) since it is directly proportional to energy. Note that wavenumbers are often called frequencies, although strictly it is incorrect. However, it is not a serious error as long as we keep in mind that and ν = c/λ. The ordinary infrared region 2.5–15 μ (4000–667 cm−1) is of greatest practical use to organic chemists. The region 0.8–2.5 μ (12,500–4000 cm−1) is called the near infrared and the region 15–200 μ (667–50 cm−1) the far infrared. The absorption of infrared radiation by a molecule occurs due to quantized vibrational and rotational energy changes when it is subjected to infrared irradiation. Thus, IR spectra are often called vibrational-rotational spectra.