Brillouin light scattering is the result of the interaction between a photon of light and the acoustic phonons of a material. This interaction results in two scattering events: Stokes scattering and anti-Stokes scattering. These are respectively attributed to the creation and the annhilation of phonons of wavevector, q, and frequency, Omega, where by conservation of momentum and energy:

where k is the incident or scattering wavevector and w is the incident or scattering frequency.

This can be graphically represented as is shown below where the wavy line indicates the created or annhilated phonon:

The measured freuency shift of the light is equal to that of the phonon and it's wave vector (q) can be found from the above equations and is given by:

Thus, the Brillouin longitudinal frequency shift as related to the longitudinal sound velocity is given by: