Chapter 30 Quantum Physics30.1 Blackbody Radiation and Planck’s Hypothesis of Quantum Energy 30.2 Photons and the Photoelectric Effect30.3 The Mass and Momentum of a Photon30.4 Photon Scattering and Compton Effect30.5 The de Broglie Hypothesis and Wave-Particle Duality30.6 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, theHeisenberg 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 anelectron microscope. In this case the electrons are behaving likewaves.Section 30.1: Objects that are hot appear different colors depending on how hot they are: Dim Red - hotYellow - hotterWhite hot - very hot****BlackbodyA perfect blackbody or, simply, blackbody is used when referring to an object that absorbs all the electromagnetic waves falling on it.Plank's constantAt 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 quantizedenergies. Plank's quantized energies are given by E = nhf, where n = 0, 1, 2, 3,..., his the Plank's constant ( 6.63 x 10-34J.s ), and f is the vibration frequency.For a black body, max intensity radiated blue shifts as temperature increases.Red hot is not as hot as blue hot. 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 theradiation that it emits.E = nhf, n=1,2,3 . . .= n(hc / λ)Electromagnetic wave of energy E=hfTo 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.Ei=2hfEf=hfFigure 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 quantizedEn= nhf, n=1,2,3 . . .h – Planck’s Constant = 6.63x10-34JsLight is emitted in discrete “chunks” called PHOTONSEnergy of a photon, E = hfThe suggestion is that light is a little particleof energy BUT the amount of energy is related to the frequency.PARTICLES AND WAVES ARE RELATED!****PhotonsAll electromagnetic radiation consists of photons, which are packets of energy. The energy of a photon is E = hf, where his Plank's constant and fis the frequency of the light. A photon in a vacuum always travels at the speed of light cand has no mass.Photoelectric effectThe photon-electric effect is the phenomenon in which light shining on a metal surface causes electrons to be ejected from the surface.Work functionThe work function W0of 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 KEmaxthat is related to the energyhfof the incident photon byhf=KEmax+ W0Since 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 whenyou shine light on themMetal surfaceLighte-If light is a wave you expect that very intense light will result inthe 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=f0the 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: characteristics1 Photocurrent α intensity2 KE of photoelectrons α frequency but not intensity.3 Below fothere 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.Metal surfaceLight, E=hfKEe-=1/2 mev2Work function, W0max 020012oKE hf Whchf W