Raman scattering is the inelastic scattering of a photon. Most photons are elastically scattered from atoms or molecules ( Rayleigh scattering), such that the scattered photons have the same energy and wavelength as the incident photons. However, a small fraction of scattered light is absorbed and subsequently emitted via an intermediate electronic state, with the scattered photons having a frequency different from the frequency of the incident photons. The energy differences are equal to the differences of the vibrational and rotational energy levels of the molecule. There are two types of Raman scattering: (1) Stokes scattering, when the molecule absorbs energy and the resulting photon has a lower energy and is shifted to the red side of the incident spectrum, and (2) anti-Stokes scattering, where the molecule loses energy and the resulting photons are more energetic and thus shifted to the blue side of the spectrum.