Electromagnetic radiation: light and other forms of radiant energy. Wavelength ( ): distance between two consecutive identical points on a wave. Frequency (v): number of full cycles of wave that pass a point in a second. Hertz (hz): unit in which wave frequency is reported, s-1. Wavelength and frequency are inversely proportional and we can calculate one from the other from the relationship: e = hv. High-energy radiation corresponds to short wavelengths and vice versa: ultraviolet light (higher energy) has shorter wavelength. Infrared radiation (lower energy) has a longer wavelength. Experimental process of measuring which frequencies of radiation are absorbed or emitted by a substance and correlating these patterns with details of molecular structure. We use a portion of the electromagnetic spectrum called vibrational infrared region (extends from 4000 to 400 cm-1) Wavenumber: characteristic of electromagnetic radiation equal to the number of waves per cm: higher wavenumber = higher energy of radiation.