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Why are lattice vibrations called optical and acoustic?

Here you will learn why the two dispersion branches (of a diatomic basis) are called acoustic and optical.
Answer #1
Level 3
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.