A type of luminescence, fluorescence is the emission of visible light by a substance as a result of prior absorption of light or electromagnetic radiation. When a molecule is exposed to light or radiation, its valence electrons absorb the energy and jump to a higher energy state. When an electron relaxes back to its ground state, it emits a photon with a longer wavelength than the original light or radiation absorbed. A molecule that displays fluorescent properties is known as a fluorophore. Fluorophores will have specific excitation and emission ranges and play roles in a wide variety of applications such as lighting, analytical chemistry, spectroscopy, biochemistry, medicine, microscopy, and forensics. Fluorescence offers sensitivities as low as parts per trillion because often only one excitation wavelength is used and only one emission wavelength is detected, offering incredibly high selectivity.