<P> The emittance of an object quantifies how much light is emitted by it . This may be related to other properties of the object through the Stefan--Boltzmann law . For most substances, the amount of emission varies with the temperature and the spectroscopic composition of the object, leading to the appearance of color temperature and emission lines . Precise measurements at many wavelengths allow the identification of a substance via emission spectroscopy . </P> <P> Emission of radiation is typically described using semi-classical quantum mechanics: the particle's energy levels and spacings are determined from quantum mechanics, and light is treated as an oscillating electric field that can drive a transition if it is in resonance with the system's natural frequency . The quantum mechanics problem is treated using time - dependent perturbation theory and leads to the general result known as Fermi's golden rule . The description has been superseded by quantum electrodynamics, although the semi-classical version continues to be more useful in most practical computations . </P> <P> When the electrons in the atom are excited, for example by being heated, the additional energy pushes the electrons to higher energy orbitals . When the electrons fall back down and leave the excited state, energy is re-emitted in the form of a photon . The wavelength (or equivalently, frequency) of the photon is determined by the difference in energy between the two states . These emitted photons form the element's spectrum . </P> <P> The fact that only certain colors appear in an element's atomic emission spectrum means that only certain frequencies of light are emitted . Each of these frequencies are related to energy by the formula: </P>

An elements unique colors produced when electrons are in the excited state are called its