Spectroscopy is the study of the interaction of matter and electromagnetic radiation. A continuum of different types of electromagnetic radiation is associated w/ particular energy range > electromagnetic spectrum. Frequency is the number of wave crests that pass by a given point in one second; measured in hertz (hz) Wavelength is the distance from any point on one wave to the corresponding point on the next wave; measured in nanometers. Wavenumber is the number of waves in cm^-1. A stretch is a vibration occurring along the line of the bond; stretching vibration changes the bond length. A bend is a vibration that does not occur along the line of the bond; a bending vibration changes the bond angle. A diatomic molecule (eg. h-cl) undergoes only a stretching vibration b/c it has no bond angles. Molecules w/ 3+ atoms can include both stretches and bends.