Brillouin scattering, named for Léon Brillouin, occurs when light in a medium (such as water or a crystal) interacts with time dependent optical density variations and changes its energy (frequency) and path. The density variations may be due to acoustic modes, such as phonons, magnetic modes, such as magnons, or temperature gradients. As described in classical physics, when the medium is compressed its index of refraction changes and the light's path necessarily bends.