Physics 102: Lecture 22, Slide 1 Hour Exam 3 Monday, April. 18 (one week from today!) Lectures 14 21 Homework through HW 11 Discussions through Disc 11 Review session Sunday, Apr. 17, 3pm, 141 Loomis Will cover Fall 10 exam 3 Sign up for conflict exam by Thursday Physics 102: Lecture 22, Slide 2 Quantum Mechanics: Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality Physics 102: Lecture 22 Physics 102: Lecture 22, Slide 3 State of Late 19 th Century Physics Two great theories Newtons laws of mechanics, including gravity Maxwells theory of electricity & magnetism, including propagation of electromagnetic waves Butsome unsettling experimental results calls into question these theories Einstein and relativity The quantum revolution Lecture 28 Lectures Classical physics Physics 102: Lecture 22, Slide 4 Quantum Mechanics! At very small sizes the world is VERY different! Energy is discrete, not continuous. Everything is probability; nothing is for certain. Particles often seem to be in two places at same time. Looking at something changes how it behaves. Physics 102: Lecture 22, Slide 5 Three Early Indications of Problems with Classical Physics Blackbody radiation Photoelectric effect Wave-particle duality Physics 102: Lecture 22, Slide 6 Hot objects glow (toaster coils, light bulbs, the sun). As the temperature increases the color shifts from Red (700 nm) to Blue (400 nm) The classical physics prediction was completely wrong! (It said that an infinite amount of energy should be radiated by an object at finite temperature) Blackbody Radiation Physics 102: Lecture 22, Slide 7 Blackbody Radiation Spectrum Visible Light: ~0.4 m to 0.7 m Higher temperature: peak intensity at shorter Wiens Displacement Law: max T = 2.898x10 -3 mK Physics 102: Lecture 22, Slide 8 Blackbody Radiation: First evidence for Q.M. Max Planck found he could explain these curves if he assumed that electromagnetic energy was radiated in discrete chunks, rather than continuously. The quanta of electromagnetic energy is called the photon. Energy carried by a single photon is E = hf = hc/ Plancks constant: h = x Joule sec Physics 102: Lecture 22, Slide 9 Preflights 22.1, 22.3 A series of light bulbs are colored red, yellow, and blue. Which bulb emits photons with the most energy? The least energy? Which is hotter? (1) stove burner glowing red (2) stove burner glowing orange Blue! Lowest wavelength is highest energy. E = hf = hc/ Red! Highest wavelength is lowest energy. Hotter stove emits higher-energy photons (lower wavelength = orange) 80% correct! Physics 102: Lecture 22, Slide 10 Nobel Trivia For which work did Einstein receive the Nobel Prize? 1) Special RelativityE=mc 2 2) General Relativity Gravity bends Light 3) Photoelectric Effect Photons 4) Einstein didnt receive a Nobel prize. Physics 102: Lecture 22, Slide 11 Photoelectric Effect Light shining on a metal can knock electrons out of atoms. Light must provide energy to overcome Coulomb attraction of electron to nucleus Light Intensity gives power/area (i.e. Watts/m 2 ) Recall: Power = Energy/time (i.e. Joules/sec.) metal light ee Physics 102: Lecture 22, Slide 12 Photoelectric Effect: Light Intensity What happens to the rate electrons are emitted when increase the brightness? What happens to max kinetic energy when increase brightness? Rate increases Nothing metal light ee Physics 102: Lecture 22, Slide 13 Photoelectric Effect: Light Frequency What happens to rate electrons are emitted when increase the frequency of the light? What happens to max kinetic energy when increase the frequency of the light? Increases Nothing, but goes to 0 for f < f min metal ee light No e ee Physics 102: Lecture 22, Slide 14 Photoelectric Effect Summary Each metal has Work Function (W 0 ) which is the minimum energy needed to free electron from atom. Light comes in packets called Photons E = h f h = x Joule sec Maximum kinetic energy of released electrons K.E. = hf W 0 hf W0W0 KE ee Physics 102: Lecture 22, Slide 15 ACT: Photon A red and green laser are each rated at 2.5mW. Which one produces more photons/second? 1) Red2) Green3) Same Red light has less energy/photon so if they both have the same total power, red has to have more photons/time! Physics 102: Lecture 22, Slide 16 Quantum Physics and the Wave- Particle Duality I. Is Light a Wave or a Particle? Wave Electric and Magnetic fields act like waves Superposition: Interference and Diffraction Particle Photons (blackbody radiation) Collision with electrons in photo-electric effect BOTH Particle AND Wave Physics 102: Lecture 22, Slide 17 II. Are Electrons Particles or Waves? Particles, definitely particles. You can see them. You can bounce things off them. You can put them on an electroscope. How would know if electron was a wave? Look for interference! Physics 102: Lecture 22, Slide 18 Youngs Double Slit w/ electron Screen a distance L from slits Source of monoenergetic electrons d 2 slits- separated by d L Jnsson 1961 Physics 102: Lecture 22, Slide 19 Electrons are Waves? Electrons produce interference pattern just like light waves. Need electrons to go through both slits. What if we send 1 electron at a time? Does a single electron go through both slits? Physics 102: Lecture 22, Slide 20 Youngs Double Slit w/ electron Source of monoenergetic electrons d L Merli 1974 Tonomura 1989 Same pattern for photons One electron at a time Interference pattern = probability Physics 102: Lecture 22, Slide 21 ACT: Electrons are Particles If we shine a bright light, we can see which hole the electron goes through. Does the electron pass through... (1) Both Slits(2) Only 1 Slit But now the interference is gone! Physics 102: Lecture 22, Slide 22 Electrons are Particles and Waves! Depending on the experiment electron can behave like wave (interference) particle (localized mass and charge) If we dont look, electron goes through both slits. If we do look it chooses 1. Im not kidding its true! Physics 102: Lecture 22, Slide 23 Schrdinger's Cat Place cat in box with some poison. If we dont look at the cat it will be both dead and alive! Poison Physics 102: Lecture 22, Slide 24 More Nobel Prizes! 1906 J.J. Thompson Showing cathode rays are particles (electrons) G.P. Thompson (JJs son) Showed electrons are really waves. Both were right! Physics 102: Lecture 22, Slide 25 Quantum Summary Particles act as waves and waves act as particles Physics is NOT deterministic Observations affect the experiment Physics 102: Lecture 22, Slide 26 See you Wednesday!