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 h ν ν that is proportional to the frequency ν ν of the corresponding electromagnetic wave. The proportionality constant h 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 h ν ν , the highest kinetic energy K max is K max = h ν ν − − W . Here, W 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 Φ Φ or φ φ . If the work function is written as W = h ν ν o , the formula for the maximum kinetic energy of the ejected electrons becomes K max = h ( ν ν − − ν ν o ) .