1. The mass–energy relationship announced by Einstein in 1905 in the form E=mc2, where E is a quantity of energy, m its mass, and c is the speed of light. It presents the concept that energy possesses mass. See also relativity.
2. The relationship Emax=hf−W, where Emax is the maximum kinetic energy of electrons emitted in the photoemissive effect, h is the Planck constant, f the frequency of the incident radiation, and W the work function of the emitter. This is also written Emax=hf−ϕe, where e is the electronic charge and ϕ a potential difference, also called the work function. (Sometimes W and ϕ are distinguished as work function energy and work function potential.) The equation can also be applied to photoemission from gases, when it has the form: E=hf−I, where I is the ionization potential of the gas.