The photoelectric effect refers to the notion of emission of electrons from the surface of a conductor when light strikes it. Hertz was the first to observe the photoelectric effect in 1887. Albert Einstein analyzed and explained the photoelectric effect in 1905 using the notion of quantum physics, since the classical wave theory of light could not explain the photoelectric effect. Einstein postulated that a beam of light consists of small bundles of energy called quanta, or photons. The energy E of a quantum is proportional to its frequency ν E = hν Where h = 6.63*10-34Js is a universal constant known as Planck's constant. If an electron gets energy that is greater than work functionA, it can escape from the surface. According to Einstein, the maximum kinetic energy for an emitted electron is determined by the following equation hν=A+(mv2max/2) . This equation explains the basic features of the photoelectric effect: 1. The number of electrons emitted from the surface per second is proportional to the radiation flux. 2. The maximum kinetic energy of photoelectrons increases in linear progression with the frequency ν, and it does not depend on the influx of incident light. If there is an electric field between the cathode and the anode of a phototube that is pushing the electrons back to the anode, the electron flow stops when the reverse potential reaches the stopping potentialU0, which is given by mv2max/2 = eU0. No photoelectrons are emitted unless the frequency of light is greater than the minimum value, known as threshold frequency νmin νmin=A/h , or λmax=hc/A. Alkaline metals have their threshold frequency in the region of visible light.