So I see them in the books labelled as accoustic and optical phonons but I don't seem to find a comprehensive treatment of the matter for a beginner who doesn't know a thing about the dispersion curves. I'd prefer not to dwell too much into the mechanical treatment if possible since I just need a basic understanding instead of an extensive one. Is there a book or a web resource maybe which can help a beginner in getting the hang of these processes. Also, it'd be helpful if the resource also discusses the correlation of these dispersion curves with optical phenomena. Thank you in advance for your inputs.