When light is scattered from an atom or a molecule, most photons are elastic scattering (Rayleigh scattering), such that the scattered photons have the same energy and wavelength as the incident photons have. In contrast to Rayleigh scattering, Raman scattering is inelastic scattering and has two different types depending on energy state: Stokes scattering and anti-Stokes scattering. The interaction of light with matter in a linear regime allows the absorption of the simultaneous emission of light precisely matching the difference in energy levels of the interacting electrons. Stokes scattering happens when a molecule absorbs energy, and anti-Stokes scattering happens when a molecule loses energy.