PHOTOELECTRIC EFFECT WHAT DOES PHOTOELECTRIC EFFECT MEANS?? The phenomenon of emission of electrons from metal under the effect of light is called photoelectric effect. The electrons emitted by photoelectric effect are called photoelectrons and the current produced is called photoelectric effect. In this effect the light of short wavelength is more effective. HISTORY First observed by Hertz in 1887 After one year , Hallwachs confirmed by an experiment by using zinc plates. In 1898 J.J. Thompson established When light falls on a metal surface , electrons are emitted from the surface. Finally understood by Lenard in 1900 Various experiments were conducted by Lenard and Millikan. THE PHENOMENON DEPENDENCE ON VARIOUS FACTORS 1. Light ray of definite intensity fall on plate P , photons are emitted. 2. plate C is positively charged, all these photons reach plate C. 3. Increase in positive potential does not results in increase in number of photons. 4. Increase in intensity of light results in increase of photons. 5. Hence increase in the current. 1. If at stopping potential , light of frequency higher than before be made to fall on P , current is established. 2. Higher the frequency higher will be the maximum kinetic energy. 3. Lowest frequency which can emit photons from a material is called threshold frequency or cut-off frequency of that material. In 1905 Einstein explained photoelectric theory . The intensity of light depends on the number of these photons. When photons fall on a metal, it transfers “whole” of its energy to any “one” of the electron present in the metal and its own existence is vanished. Energy Of a Photon 1) 2) 3) 4) 5) Rest mass = zero, but kinetic mass not zero. Photon travels with the speed of light. Einstein's mass energy relation (E=mc ˆ2) Energy of photon E= hv/c^2 ADVANTAGES: Used in turning on motor. Used for switching on & off street . Used in cameras . Used in industries. Used in solar cells for conversion of light energy into electric energy.