nergy hf is required to emitan electron from the metal. Light with a frequencygreater than the threshold frequency (fo) has moreenergy than required to emit an electron. Theexcess energy again becomes the kinetic energy ofthe electron, thus, Ek = hf - hfo. This equation isknown as Einstein’s Photoelectric Equation. Anelectron cannot accumulate photons until it hasenough energy to break free; only one photon caninteracts with one electron at a time. In Einstein’sequation hfo, is actually the minimum energyrequired to free an electron. Not all electrons in asolid have the same energy; most need more thenthe minimum (hfo) to escape. Therefore, thekinetic energy of the emitted electrons is actuallythe maximum kinetic energy an emitted electroncould have. Einstein’s theory can be tested byindirectly measuring the kinetic energy of theemitted electrons. A variable electric potentialdifference across the tube makes the anodenegative. Since, the anode rejects the emittedelectrons from the cathode, the electrons musthave sufficient kinetic energy at the cathode toreach the anode before turning back. A light ofmeasurable frequency f, is directed at the cathode.An ammeter measures the current flowing throughthe circuit. As the opposing potential difference isincreased, the anode is made increasingly morenegative. At some voltage, called the stoppingpotential, there is a zero reading from the ammeterbecause the electrons do not reach the anode.This is due to an insufficient amount of suppliedenergy to the electrons. The maximum kineticenergy of the electrons at the cathode equals theirpotential energy at the anode. Emax = -qVo,where Vo is the magnitude of the stoppingpotential in volts (J/C), and q is the charge of theelectron (-1.60 x 10-19C). The joule is too largea unit of energy to use with atomic systems,therefore the electron volt (eV) is used instead. 1eV = (1.60 x 10-19C) (1V) = (1.60 x 10-19C)(V). Also, 1 eV = 1.60 x 10-1...
