Ain Shams U. Faculty of Engineering Mathematics and Engineering Physics Department Lecture 2 : Photoelectric Effect د. حاتم الرفاعي سبتمبر 2004

Sept.-Dec الفزياء الهندسية – أولى كهرباء Contents Websites Historical background Photoelectric phenomena Classical physics prediction Modern physics explanation

Sept.-Dec الفزياء الهندسية – أولى كهرباء Websites oEffect/photo.htm oEffect/photo.htm ml ml ne/photoelectric.html ne/photoelectric.html

Sept.-Dec الفزياء الهندسية – أولى كهرباء Historical background 1888, Hallwachs found that when a Zinc plat is illuminated with ultraviolet light, it becomes positively charged. 1899, Thomson showed that electrons were expelled from the plate. 1902, Lenard experiment. 1905, Einstein explanation.

Sept.-Dec الفزياء الهندسية – أولى كهرباء Photoelectric effect If the incident wave frequency ( f ) is smaller than a certain value ( f o ), there is no current flows. If f> f o, the current flows instantaneously. Reverse biased connection I 2 >I 1 Forward Reverse

Sept.-Dec الفزياء الهندسية – أولى كهرباء Photoelectric effect With C forward biased, the electrons are accelerated faster resulting in an increasing current till a saturating value. The higher the incident wave intensity, the higher the saturation current. With C reverse biased, electrons are repelled from reaching C, and thus the current decreases. At a certain reverse potential - V o the current becomes zero independent on the incident wave intensity. Reverse biased connection I 2 >I 1 Forward Reverse

Sept.-Dec الفزياء الهندسية – أولى كهرباء Classical physics prediction Electrons can be emitted regardless of the incident frequency, though it will take longer time for smaller incident wave amplitude. There should be a time delay between the wave illumination and the emission of electrons. The higher the wave intensity, the higher electron energy, and thus the higher the stopping voltage.

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation The electromagnetic wave consists of many lumped energy particles called photons. The energy of each individual photon is given by Jole

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation If N is the total number of photons incident during time interval T, then the total incident optical energy is The incident energy per second (power) is given by n=N/T is the number of incident photons per second. Jole Watt = J/Sec.

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation Interaction (absorption / emission) between the electromagnetic wave and matter occurs through annihilation ( القضاء على ) / creation ( خلق ) of a quantized ( كمي ) energy (photon). In the photoelectric effect, each single absorbed photon gives its total energy ( hf ) to one single electron. This energy is used by the electron to: Overcome the attraction force of the material. Gain kinetic energy when freed from the material.

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation Work function (  ): It is the minimum required energy required by an electron to be free from the attraction force of the metal ions. Some of the electrons may need more energy than the work function to be freed. Total Energy  Zero -ve +ve The most energetic electrons in the material

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation  Total Energy Zero -ve +ve The most energetic electrons in the material hf

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation  Total Energy Zero -ve +ve The most energetic electron outside the material hf

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation The electrons that need only the work function to be freed, will have the greatest kinetic energy outside the metal. The electrons requiring higher energy to be freed, will have lower kinetic energy.

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation Thus, there is a minimum required photon energy ( hf o ) to overcome the work function of the material. If the incident photon energy is less than the work function, the electron will not be freed from the surface, and no photoelectric effect will be observed. = No photoelectric current If hf <  If f < f o

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation The most energetic electrons are stopped by the reverse biased stopping potential - V o. Kinetic energy of photoelectron = potential energy created by the reverse biased connection

Sept.-Dec الفزياء الهندسية – أولى كهرباء Modern physics explanation The stopping potential doesn ’ t depend on the incident light intensity. The stopping potential depends on the incident frequency. Slope = h/e