Difference between an optical and an acoustic phonon

[learner@123]

Hello friends.
My question consists two parts,
1-What is the difference between an optical and an acoustic phonon?
1-What are the conditions by which we can decide the type of phonon i.e optical or acoustic phonon ?
Hope to get the reply soon.

 

[marcusl]

The two branches arise in diatomic crystals where phonons can propagate in a band or branch that has small (sound or acoustic) or large (infrared, typically, hence optical) wave numbers. Have you read the wiki article on phonons? That's a good place to start. Leon Brillouin's book Wave Propagation in Periodic Structures (or something like that) has an exceptionally clear and intuitive treatment, if you want to dig further.

 

[christopher.s]

Fill a glass with water and sand. Rotate the glass about it's axis in a sinusoidal fashion. If you rotate slowly enough, the particles will move roughly in phase with the glass. If you rotate fast enough, the particles will begin to move out of phase with the glass. This is analogous to acoustic and optical phonons.