Why are lattice vibrations called optical and acoustic?

Answer #1

Answered by
Dispersion Relation (Graph) of the Lattice Vibrations of a Diatomic Crystal Lattice
Dispersion relation for a diatomic basis in 1d. Two solutions are obtained: optical and acoustic branch.

The dispersion relation \( \omega(k) \) of a one-dimensional crystal lattice with a diatomic basis has two branches. These branches result from the solution of two differential equations for this problem of diatomic chains.

The one branch (one solution) \(\omega_-(k) \) is called acoustic because in this case the lattice planes oscillate in phase, as is the case of acoustic waves.

The other branch (the other solution) \(\omega_+(k) \) is called optical because this solution of the respective differential equation gives an opposite-phase oscillation of the lattice planes (with \(m_1\) and \(m_2\)). For example, if the atoms are ions, then they are electrically charged. An antiphase oscillation creates electric dipole moments in the crystal, which affects the optical properties of the crystal.

+ Perfect for high school and undergraduate physics students
+ Contains over 500 illustrated formulas on just 140 pages
+ Contains tables with examples and measured constants
+ Easy for everyone because without vectors and integrals

Learn more