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Brillouin scattering is inelastic scattering of light involving acoustic phonons or the corresponding traveling elastic fluctuations. The scattered photon exchanges energy and momentum with the disturbance, producing a small shift from the incident frequency. For an isotropic medium with sound speed $v$, refractive index $n$, vacuum wavelength ${\lambda }_0$ and scattering angle $\theta$, the shift magnitude is approximately $\Delta \nu =(2nv/{\lambda }_0)\sin (\theta /2)$.

A narrow-line laser illuminates a sample, collection optics select a scattering angle, and a high-resolution Fabry–Pérot interferometer separates weak shifted components from strong nearly unshifted scattering. Tandem interferometers improve rejection of the central component. With known geometry and refractive index, the shift measures an acoustic speed and constrains elastic properties.^[ref]