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Chapter 30 Quantum Physics 30 1 Blackbody Radiation and Planck s Hypothesis of Quantum Energy 30 2 Photons and the Photoelectric Effect 30 3 The Mass and Momentum of a Photon 30 4 Photon Scattering and Compton Effect 30 5 The de Broglie Hypothesis and WaveParticle Duality 30 6 The Heisenberg Uncertainty Principle The wave particle duality nature will be discussed in this chapter We will learn that a wave can exhibit particle like characteristics and a particle can exhibit wave like characteristics We will also see that light and other electromagnetic waves can exhibit some of the same characteristics that are normally associated with particles In particular we will find that electromagnetic waves can be regarded as being composed of discrete packets of energy called photons and that a photon has momentum just as a particle does This chapter concludes with an introduction to one of the most unusual scientific principle the Heisenberg uncertainty principle which places a limit on our knowledge of certain aspect of the physical world Light waves can behave like particles Particles like electrons can behave like waves The picture of a fly at the start of the chapter was taken using an electron microscope In this case the electrons are behaving like waves Section 30 1 Objects that are hot appear different colors depending on how hot they are Dim Red hot Yellow hotter White hot very hot Blackbody A perfect blackbody or simply blackbody is used when referring to an object that absorbs all the electromagnetic waves falling on it Plank s constant At a constant temperature a perfect blackbody absorbs and reemits all the electromagnetic radiation that falls on it Max Plank calculated the emitted radiation intensity per unit wavelength as a function of wavelength In his theory Plank assumed that a blackbody consists of atomic oscillators that can have only quantized energies Plank s quantized energies are given by E nhf where n 0 1 2 3 h is the Plank s constant 6 63 x 10 34 J s and f is the vibration frequency A Black Body absorbs all the light that hits it NO light is reflected that is why it appears black It does not need to be black For example a furnace If all radiation is absorbed then it also is very effective at giving off or emitting radiation That is heat the black body to some temperature and measure the radiation that it emits For a black body max intensity radiated blue shifts as temperature increases Red hot is not as hot as blue hot E nhf n 1 2 3 n hc Ei 2hf Ef hf Electromagnetic wave of energy E hf To emit a single very high frequency photon requires a lot of energy How much energy in a photon of blue light at 400 nm Express your result in eV Figure 30 4 The photon model of light In the photon model of light a beam of light consists of many photons each with an energy hf The more intense the beam the more tightly packed the photons Emitted Energy is quantized En nhf n 1 2 3 h Planck s Constant 6 63x10 34 Js Light is emitted in discrete chunks called PHOTONS Energy of a photon E hf The suggestion is that light is a little particle of energy BUT the amount of energy is related to the frequency PARTICLES AND WAVES ARE RELATED Photons All electromagnetic radiation consists of photons which are packets of energy The energy of a photon is E hf where h is Plank s constant and f is the frequency of the light A photon in a vacuum always travels at the speed of light c and has no mass Photoelectric effect The photon electric effect is the phenomenon in which light shining on a metal surface causes electrons to be ejected from the surface Work function The work function W0 of a metal is the minimum work that must be done to eject an electron from the metal In accordance with the conservation of energy the electrons ejected from a metal have a maximum kinetic energy KEmax that is related to the energy hf of the incident photon by hf KEmax W0 Since everyone thought light was a wave the theory was not initially accepted Light has particle like properties and wave like properties This is difficult to accept However the theory was able to explain many other phenomena The photoelectric effect Some metals give of electrons when you shine light on them Light Metal surface eIf light is a wave you expect that very intense light will result in the liberation of lots of electrons and that this will be independent of the frequency of the light Figure 30 5 The photoelectric effect The photoelectric effect can be studied with a device like that shown Light shines on a metal plate ejecting electrons which are then attracted to a positively charged collector plate The result is an electric current that can be measured with an ammeter Figure 30 6 The kinetic energy of photoelectrons The maximum kinetic energy of photoelectrons as a function of the frequency of light Note that sodium and gold have different cutoff frequencies as one might expect for different materials On the other hand the slope of the two lines is the same h as predicted by Einstein s photon model of light If the frequency of the light hitting the photoelectric material is varied Below a certain frequency fo NO electrons are emitted by the material NO MATTER WHAT THE LIGHT INTENSITY IS For f f0 the emitted electrons have no KE Increasing the intensity does not increase the KE of the electrons It does increase the number of electrons emitted however Increasing f will increase the electron KE The photoelectric effect characteristics 1 Photocurrent intensity 2 KE of photoelectrons frequency but not intensity 3 Below fo there is no photoelectron emission Even for extremely intense light 4 Photoemission is immediate even for extremely low intensity light The latter three observations CANNOT be predicted by considering light to be a wave Think about a weak and a strong person trying to move a rock Millions of weak people one at a time will never move the rock however a single strong person will easily move the rock Einstein explained the photoelectric effect by using the particle nature of light Photons of discrete energy can be used provide energy to electrons to overcome the attractive metallic forces Any excess energy of the photon is transferred to KE of the electron Note All of the energy of the photon is used up Light E hf Metal surface KEe 1 2 mev2 Work function W0 KEmax hf W0 hc 1 hf W0 mv 2 2 W fo 0 h Different for different metals Questions 1 The photons emitted by a source of light do NOT all have the same energy Is the source


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