Example: Symmetric stretches do not show up in IR spectra because they involve no net change in dipole movement.

Note ↑: Raman spectroscopy occurs as a result of a molecular vibration causing a "change in polarizability" of the molecule. In contrast, for a molecule to be infrared active, the vibration must cause a change in the permanent dipole moment. A simple case of a Raman Active molecule would be a species such as CS₂. The symmetrical stretch out and then in (pictured to the right) will be detected by Raman spectroscopy. Since the molecule has no permanent dipole, this stretch would be invisible in an infrared spectra, represented in the diagram ↓.