Lect22 handout
- 1. Quantum Mechanics: Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality Physics 102: Lecture 22
- 2. State of Late 19 th Century Physics Two great theories Newton’s laws of mechanics, including gravity Maxwell’s theory of electricity & magnetism, including propagation of electromagnetic waves But…some unsettling experimental results calls into question these theories Einstein and relativity The quantum revolution Lecture 28 Lectures 22-25 “ Classical physics”
- 3. 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.
- 4. Three Early Indications of Problems with Classical Physics Blackbody radiation Photoelectric effect Wave-particle duality
- 5. Blackbody Radiation 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)
- 6. Blackbody Radiation Spectrum Visible Light: ~0.4 m to 0.7 m Higher temperature: peak intensity at shorter Wien’s Displacement Law: max T = 2.898x10 -3 m·K
- 7. 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 / Planck’s constant: h = 6.626 x 10 -34 Joule sec
- 8. 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
- 9. ACT: Nobel Trivia For which work did Einstein receive the Nobel Prize? 1) Special Relativity E=mc 2 2) General Relativity Gravity bends Light 3) Photoelectric Effect Photons 4) Einstein didn’t receive a Nobel prize.
- 10. 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 e –
- 11. 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 e –
- 12. 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 e – light No e – e –
- 13. 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 = 6.626 x 10 -34 Joule sec Maximum kinetic energy of released electrons K.E. = hf – W 0 e – hf W 0 KE
- 14. ACT: Photon A red and green laser are each rated at 2.5mW. Which one produces more photons/second? 1) Red 2) Green 3) Same
- 15. 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
- 16. 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!
- 17. Young’s Double Slit w/ electron Screen a distance L from slits Source of monoenergetic electrons L Jönsson – 1961 d 2 slits-separated by d
- 18. 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?
- 19. Young’s Double Slit w/ electron Source of monoenergetic electrons L Merli – 1974 Tonomura – 1989 Same pattern for photons One electron at a time Interference pattern = probability d
- 20. ACT: Electrons are Particles If we shine a bright light, we can ‘see’ which hole the electron goes through. (1) Both Slits (2) Only 1 Slit
- 21. Electrons are Particles and Waves! Depending on the experiment electron can behave like wave (interference) particle (localized mass and charge) If we don’t look, electron goes through both slits. If we do look it chooses 1. I’m not kidding it’s true!
- 22. Schrödinger's Cat Place cat in box with some poison. If we don’t look at the cat it will be both dead and alive! Poison
- 23. More Nobel Prizes! 1906 J.J. Thompson Showing cathode rays are particles (electrons). 1937 G.P. Thompson (JJ’s son) Showed electrons are really waves. Both were right!
- 24. Quantum Summary Particles act as waves and waves act as particles Physics is NOT deterministic Observations affect the experiment
Editor's Notes
- #2: 1
- #6: Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared.
- #7: Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared. Classical theory at 3000 k: ultraviolet catastrophe (see p. 985 text)
- #8: Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared.
- #12: greater intensity increases current does not change maximum KE
- #13: higher frequency light increases max. KE Below threshold freq, no current Electrons emitted immediately, no delay as “energy is accumulated”
- #14: All puzzles explained with quantum theory.
- #18: Go to physics 2000 web site for JAVA version
- #20: Go to physics 2000 web site for JAVA version