Quantum Mechanics: Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality

1. Physics 102: Lecture 22 Quantum Mechanics:Blackbody Radiation, Photoelectric Effect, Wave-Particle Duality

2. opposite! Recap. • Interference: Coherent waves • Full wavelength difference = Constructive • ½ wavelength difference = Destructive • Multiple Slits • Constructive d sin(q) = m l (m=1,2,3…) • Destructive d sin(q) = (m + 1/2) l 2 slit only • More slits = brighter max, darker mins • Huygens’ Principle: Each point on wave front acts as coherent source and can interfere. (see Lab 8) • Single Slit: • Destructive: w sin(q) = m l (m=1,2,3…) • Resolution: Max from 1 at Min from 2

3. State of Late 19th 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

5. 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. • If you aren’t confused by the end of this lecture, you weren’t paying attention!

6. Three Early Indications of Problems with Classical Physics • Blackbody radiation • Photoelectric effect • Wave-particle duality

7. Blackbody Radiation Hot objects glow (toaster coils, light bulbs, the sun). As the temperature increases the color shifts from Red to Blue. The classical physics prediction was completely wrong! (It said that an infinite amount of energy should be radiated by an object at finite temperature.)

9. Visible Light: ~0.4mm to 0.7mm Blackbody Radiation Spectrum Higher temperature: peak intensity at shorter l

10. 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/l Planck’s constant: h = 6.626 X 10-34 Joule sec

11. 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? Blue! Lowest wavelength is highest energy. E = hf = hc/l Red!Highest wavelength is lowest energy. Which is hotter? (1) stove burner glowing red (2) stove burner glowing orange

13. 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

14. Nobel Trivia For which work did Einstein receive the Nobel Prize? 1) Special Relativity E=mc2 2) General Relativity Gravity bends Light 3) Photoelectric Effect Photons 4) Einstein didn’t receive a Nobel prize.

15. 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/m2) • Recall: Power = Energy/time (i.e. Joules/sec.)

17. 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?

18. 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?

19. Photoelectric Effect Summary • Each metal has “Work Function” (W0) 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 – W0

21. Quantum Physics and the Wave-Particle DualityI. Is Light a Wave or a Particle? • Wave • Electric and Magnetic fields act like waves • Superposition: Interference and Diffraction • Particle • Photons • Collision with electrons in photo-electric effect BOTH Particle AND Wave

22. 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!

23. d 2 slits-separated by d Young’s Double Slit w/ electron • JAVA Source of monoenergetic electrons L Screen a distance L from slits

25. 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?

26. 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

27. 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! 46

29. 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

30. 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!

31. Quantum Summary • Particles act as waves and waves act as particles • Physics is NOT deterministic • Observations affect the experiment • (coming soon!)