In 1905, Einstein proposed a theory of the photoelectric effect using a concept that light consists of tiny packets of energy known as photons or light quanta. Each packet carries energy  that is proportional to the frequency  of the corresponding electromagnetic wave. The proportionality constant  has become known as the Planck constant. In the range of kinetic energies of the electrons that are removed from their varying atomic bindings by the absorption of a photon of energy , the highest kinetic energy   is
Here,  is the minimum energy required to remove an electron from the surface of the material. It is called the work function of the surface and is sometimes denoted  or . If the work function is written as
the formula for the maximum kinetic energy of the ejected electrons becomes
Kinetic energy is positive, and  is required for the photoelectric effect to occur. The frequency  is the threshold frequency for the given material. Above that frequency, the maximum kinetic energy of the photoelectrons as well as the stopping voltage in the experiment  rise linearly with the frequency, and have no dependence on the number of photons and the intensity of the impinging monochromatic light. Einstein's formula, however simple, explained all the phenomenology of the photoelectric effect, and had far-reaching consequences in the development of quantum mechanics.