To understand the nature of light and how it is normally created, it is necessary to study matter at its atomic level. Atoms are the building blocks of matter, and the motion of one of their constituents, the electron, leads to the emission of light in most sources.
        Light can be emitted, or radiated, by electrons circling the nucleus of their atom. Electrons can circle atoms only in certain patterns called orbitals, and electrons have a specific amount of energy in each orbital. The amount of energy needed for each orbital is called an energy level of the atom. Electrons that circle close to the nucleus have less energy than electrons in orbitals farther from the nucleus. If the electron is in the lowest energy level, then no radiation occurs despite the motion of the electron. If an electron in a lower energy level gains some energy, it must jump to a higher level, and the atom is said to be excited. The motion of the excited electron causes it to lose energy, and it falls back to a lower level. The energy the electron releases is equal to the difference between the higher and lower energy levels. The electron may emit this quantum of energy in the form of a photon.
        Each atom has a unique set of energy levels, and the energies of the corresponding photons it can emit make up what is called the atom's spectrum. This spectrum is like a fingerprint by which the atom can be identified. The process of identifying a substance from its spectrum is called spectroscopy. The laws that describe the orbitals and energy levels of atoms are the laws of quantum theory. They were invented in the 1920s specifically to account for the radiation of light and the sizes of atoms.