An optical phonon branch has nonzero frequency as the wavevector tends to zero. With more than one atom in a primitive cell, relative internal displacement can change bond lengths even when neighboring cells move in phase, producing a restoring force at zero wavevector.
A diatomic chain illustrates the contrast: its acoustic branch approaches a common translation, whereas its optical branch has relative motion within the cell. The name does not guarantee that every such mode couples directly to light. Infrared absorption requires an appropriate oscillating dipole, and Raman measurements have their own symmetry conditions. Branch frequency and polarization therefore describe the vibration, while optical activity requires additional information about charges and symmetry.
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