Sometimes, a photon incident on a sample is absorbed, causing an electron to jump from its ground state to a higher, excited, energy state. Around 10^-8seconds later, the electron returns to the ground state resulting in an emission of a photon of a different wavelength than the incident photon. This return to the ground state also results in an excited vibrational state which reaches equilibrium in about 10^-12 seconds. Altogether, these events are called luminescence. There are two types of luminescence that are distinguished by their ground and excited states: fluorescence and phosphorescence. The incident photons associated with luminescence are called the "excitation source". Luminescence and inelastic scattering are different and separate events even though it is difficult to differentiate the resulting photons. Except for highly viscous liquids, fluorescence of solutions is always unpolarized even if the excitation source is highly polarized. The wavelength of the fluorescing photons is always longer than the excitation source. The ratio of the energies of fluorescing photons to excitation source is typically less than 0.01.