The Particle Nature Of Light-Classical Physics-Handouts, Lecture notes of Classical Physics

Download The Particle Nature Of Light-Classical Physics-Handouts and more Lecture notes Classical Physics in PDF only on Docsity! PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 106 Summary of Lecture 34 – THE PARTICLE NATURE OF LIGHT 1. In the previous lecture I gave you some very strong reasons to believe that light is waves. Else, it is impossible to explain the interference and diffraction phenomena that we see in innumerable situations. Interference from two slits produces the characteristic pattern. 2. Light is waves, but waves in what? of what? The thought that there is some invisible medium (given the name ) turned out to be wrong. Light is actually electric and magnetic waves that aether can travel through empty space. The electric and magnetic waves are perpendicular to each other and to the direction of travel (here the z direction). 3. Electromagnetic waves transport linear momentum and energy. If the energy per unit volume in a wave is then it is carrying momentum , where / . Waves with large amplitude carry more energy and momentum. For the sun's light on earth the momentum is rather small (although it is very large cl U p p U c= ose to or inside the sun). Nevertheless, it is easily measureable as, for example, in the apparatus below. Light strikes a mirror and rebounds. -6 2 Thus the momentum of the light changes and this creates a force that rotates the mirror. The force is quite small - just 5 10 Newtons per unit area (in metre ) of the mirror.× x z y docsity.com PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 107 packet of light with energy (or )hν ω light is made of photons 4. So strong was the evidence of light as waves that observation of the photoelectric effect came as a big shock to everybody. In the diagram below, light hits a metal surface and and knocks out electrons that travel to the anode. A current flows only as long as the light is shining. Above the threshold frequency, the number of electrons ejected depends on the intensity of the light. This was called the photoelectric effect. The following was observed: a) The photoelectric effect does not occur for all frequencies ; it does not occur at all when is ν ν below a certain value. But classically (meaning according to the Maxwell nature of light as an electromagnetic wave) electrons should be ejected at any ν. If an electron is shaken vio b) It is observed that the first photoelectrons are emitted instantaneously. lently enough by the wave, it should surely be ejected! But classically the first photoelectrons should be emitte 5. Explanation of the photoelectric puzzle came from Einstein, for wh d some time after the light first strikes the surface and excites its atoms enough to cause ionization of their electrons. ich he got the Nobel Prize in 1905. Einstein proposed that the light striking the surface was actually made of little packets (called quanta in plural, quantum in singular). Each quantum has an energy -34 energy is the famous Planck's constant, 6.626 10 Joule-seconds. An electron is kicked out of the metal only when a quantum has energy and 2 2 ) where (or where h hhε π ω πν ν ε ω × = = = = (and frequency) big enough to do the job. It doesn't matter how many quanta of light - called photons - are fired at the metal. No photoelectrons will be released unless is large enough.ν Furthermore the photoelectrons are released immediately when the photon hits an electron. docsity.com