Both chemiluminescence and fluorescence emit light when an electron relaxes from a higher energy state to a lower energy state. The difference between the two phenomena lie in how the electron is excited in the first place. In chemiluminescence the electron is in a high-energy state because of the forming of an unstable intermediate in a chemical reaction. However, in fluorescence the electron is kicked up to a higher energy level by the addition of a photon, where an outside source of light at a wavelength the fluorescent molecule can absorb is required.